Hello there @Brave Sir Robin! Yes, I am aware that it may seem dubious the source of the ottoman flag, but I can garantee you it is not a fabrication and was the best flag for the Ottoman Empire I could find for the time period. Let me explain: The Zulfiqar was a muslim banner representing the legendary sword of Ali ibn Abi Talib said to have been given to him by Muhammad. It was used widely in Morocco and Turkey by Beyliks, and after, the Ottoman army (specially the Janissaries) used it as a battle flag. To represent the Ottoman Turks with that flag would be the same as represent the spanish with the Cross of Burgundy or the Confederates with the The Battle Flag of the Army of Northern Virginia.

The ottoman flag with the 3 golden crescents on a green circle on a red banner was actually the sultan's personal banner. First I started to get this idea which was suggested on this turkish wikipedia page[tr.wikipedia.org] and this other flag[tr.wikipedia.org]. Turkish triple crescent flags with swallowtails were commonly used in the empire[commons.wikimedia.org].

I saved several sources on my HD, on of which was this one image I couldn't find the original link for[i.imgur.com] but it had this description:
"Türkçesi
Yüz sene evvelki Osmanlı sancakları sağdan sola
;Selatin-i izam hazeratına mahsus(Büyük Osmanlı sultanlarına)
Osmanlı sancağı
Sağda Osmanlı sancağı solda Trablusgarb vilayetine mahsus sancak"

The flag shown there is the same shown on this arab document which I could not find the same site I couldn't find the original link, but both seem legit enough[i.imgur.com]
There is a version with a turkish translation, which can be found here[commons.wikimedia.org] and after googling it, you stumble upon
this flag on Commons[commons.wikimedia.org] which gives the translation of the phrase as "Personal Flag of the Ottoman Sultan 1518"

This flag can be seen on this flag compilation[vignette.wikia.nocookie.net] as well, which can in turn be found on this turkish wikia with an extensive discussion on the matter[tr.yenisehir.wikia.com].

Hope this will help you understand my line of thought on adopting that flag. Those, as well as the "Flaggen aller seefahrenden Nationen" by Johann Baptist Homann, were some of the many places I went to search for this one. You'd think the Ottomans, one of the most imporant empires on earth for more than 500 years would have more documentation and preserved flags. Perhaps the problem is that most historical sources are either in turkish or are not digitalized :/

In another note, great Zulfiqar flag! ^^ Could you send a link of your mod so I can check it out?

The Turkish Wikipedia page[tr.wikipedia.org] you've linked provides no source for the image of the flag in question nor does it provide a source for the claims of it being the standard of the sultan. I can not disprove your claims that the triple crescent was a common symbol within the empire. However, the flag you've linked[commons.wikimedia.org] displaying an occurance of it is an early 19th-century naval flag and gives no claims of being commonly used within the empire itself.

The image from your hard drive seems promising, but without a source or context, I would ultimately have to say it doesn't provide much confidence.

The image from commons[commons.wikimedia.org] which you've linked also provides no sources, and given that it has absolutely no usage outside of the user's upload page, I would severely doubt its historical authenticity.

After looking into the Turkish wikia page[tr.yenisehir.wikia.com] you've linked, I found the source for the image from the "Arab document". It's "The Organization of the Ottoman Army" (1907) by Mahmut Şevket Paşa. A number of excerpts can be found on this wordpress page.[thejanissaryarchives.wordpress.com]

The passage from the wikia describing the flag is this:

Originally posted by Yenisehir Wikia:

Buna paralel olarak, meşhur Sadrazam Mahmut Şevket Paşa da Osmanlı Teşkilât ve Kıyafet-i Askeriyesi adlı eserinde, "Sultan Alâeddin-i Selçukî tarafından Osman Gazi hazretlerine gönderilen alem beyaz renkte olduğu cihetle Selâtîn-i İzâm-ı Osmaniye önlerinde evailde beyaz bayrak çekilmiş ise de muahharan padişahan-ı izama mahsus olmak üzere yeşil bir zemin durumunda beyaz kılaptan ile işlenmiş üç hilâli hâvi veyahut, kırmızı bir zemin ortasında ve yeşile boyanmış bir şekl-i beyzî derununda sarı sırmayla işlenmiş birbirinin gerisinde kezalik üç hilalî muhtevi bulunan iki nevi sancak dahi isti mâl edilmiştir."

I don't speak Turkish, and I doubt you do either, so here's a (bad) Google Translation:

Originally posted by Bad Translation of Yenisehir Wikia:

Parallel to this, the famous Grand Vizier Mahmut Şevket Pasha, in his book titled "Ottoman Organization and Clothing" stated that "the white flag was taken at home in front of the cihetle Selâtîn-i İzâm-ı Osmaniye, where the kingdom of Sultan Osman Ghazi was sent by the Sultan Alâeddin-i Selçukî In the case of a green floor, in the case of a green floor, three delicacies, or two amphibious basins with three crescents in the middle of a red ground and a yellow satin in the middle of a red ground painted with yellow serpentine behind each other, It is."

This seems to be giving a written description of the flag, which could potentially be from an earlier recording. In any case, Mahmut Şevket Paşa's book does provide a more solid basis for the existance of the flag you've used. We now have two substantial historical sources of the flag, although with a significant gap of time between each publication. Furthermore, there are contesting claims for its usage as either a naval flag or an imperial standard of the sultan.

Ultimately, what image you wish to use for your mod is going to come down to your own conclusions. For now, I think I will remain with the zulfiqar flag, as I would rather have a symbol which has been conclusively proven to exist via tangible evidence rather than one which has only been attested.

As for your claims that representing the Ottoman Empire with the zulfiqar flag would be similar to using the cross of Burgundy for Spain or the battle flag the Army of Northern Virginia for the CSA, you have to look at the fundamental nature of coats of arms. Firstly, they are a tradition of western civilisation which spread throughout Christendom. Islamic nations such as the Ottomans did not traditionally utilize them. Coats of arms, as the name implies, began as designs for use on the battlefield but evolved over time into use as flags and symbols to represent dynasties and nations. Therefore, using a symbol of the battlefield to represent a nation/dynasty is not fundamentally out of the ordinary. Since the Islamic world lacks coats of arms, one has to look elsewhere for a symbol which has been associated with or which has been used to represent these dynasties and nations. I would say that a battle flag which was used for the nation's armies for several hundred years is a completely valid choice. A personal standard, as you have chosen, is another valid choice. However, one has to consider that personal standards potentially changed with each successive sultan and battle banners varied among units and over time. There's no real definitively correct choice.

As a bit of a digression, the Cross of Burgundy was a symbol which was used to represent the Duke of Burgundy or allegience to the Duke of Burgundy (a title claimed by the Spanish monarchs), and is therefore an entirely justified symbol to use to represent the Duke of Burgundy, just as their coat of arms are. Additonally, the cross of Burgundy was the official military flag of the Spanish Empire (as well as the flag of its overseas possessions) from 1492–1715 (Some sources attest to 1785) and has been seen as a supporter within the historical Spanish coat of arms since then. While I would ultimately say that the symbol is one of Burgundy rather than of Spain, five consecutive centuries of various usage within Spain does result in a significant association with Spain among the general populace, and the use of it to represent Spain as a whole, at least in a contemporary context, is not wholly unjustified despite the existance of more archetypal symbols.

As for a link to my mod, I have yet to release it. It's still a work-in-progress. It's honestly very similar to yours, though I've only discovered yours after I was already well into creating mine. (I can't understand Latin, so I tend to ignore mods with Latin titles.) WappenWiki is also my primary source for coats of arms, except I try to maintain a more uniform visual style. I'm avoiding the usage of banners/flags when a suitable coat of arms or symbol exists and adapting any items from secondary sources to match the style of WappenWiki. In cases of countries using identical symbols or very similar ones, I try to find an alternative coat of arms to utilize. For example, as the coat of arms of Trier are identical to those of Genoa, I've decided to use the coat of arms of Richard von Greiffenklau zu Vollrads, who was Archbishop-Elector of Trier from 1511 to 1531, for Trier instead. There's also a few cases where you've interpreted tags differently than I have. For example, you've used a Venetian colonial emblem for the flag of Corfu, whereas I've used the coat of arms of the County Palatine of Cephalonia and Zakynthos, as the ruler of Corfu in EU4 is the ruler of the County Palatine (although under a French version of the name), and the tag is CEP, which seems derivitive of Cephalonia.