Sorry for taking so long to notice this. I either didn't get a notification or didn't notice it among the others I was getting at the time you made this, so I've only seen this because I noticed it when reading comments on this mod lol.

First question: Absolutely. Ish. If it's modifiers and events you want the key is an on_action tied to having the perk... might want part of that on_action to include giving a flag that prevents the on_action from happening to prevent event spam.
Tyrant or Selfless: Yesn't. Just make 2 legacy tracks and make one of the conditions for both lines being to not have the first perk from the other. Making the portraits on the same legacy change doesn't seem to be possible.

One way this could be done is a fake dynasty line. Picking the first perk takes you to an event with 2 options and locks you from taking the other perks to stop anyone from wasting their renown. Both options remove the first perk and give a flag preventing you from seeing its dynasty line. One option makes it possible for you to access the tyranny line. The other option makes it possible for you to access the selfless line.
I'm not sure if removing dynasty perks is possible though... I assume it is but it might not be.

Second question: You have full permission to include the decision in your mod or make it yourself in a stand-alone, and I'm fine with it if in the time it took for me to see this you went ahead and did it anyway. I see it more as a fundamental thing for being able to play with lots of dynasty mods rather than just a thing for my mods.
I'm not sure whether or not CK3 fixed the button to open the legacies in 1.6 but can only assume they did since I'm pretty sure the amount of legacies having all DLCs would add would put it over the previous cap. If so the decision probably won't be needed but if it is you and anyone else who wants to has permission to add it to your mods or make standalone versions has permission to.
Same goes for most of my mods, where if anyone wants to take a single feature and make a separate mod for that feature, or an override that goes alongside the mod but changes something about it (like if someone wanted to override some of the values from the perks in this mod, or make titans stronger or weaker in the Eldians mod), or a mod that adds whole new things to one of my mods, to take the concept but approach it in a different way or to make an updated version of one of my mods because an update made it stop working and I'm taking too damn long to update it myself, then they can. Same goes for if someone is in my discord or otherwise finds out about the fat list of mod ideas I have and just takes an idea and makes the mod themselves.
Feel free to use my code to help with learning, or do the classic of copying and pasting it then changing parts within it to suit the mod you're making.
I make mods because I want to play the game with the things those mods add and see that nobody else has made a mod of the thing I want or if others have made it I'm either unaware or think I could do it better. I don't really care if they're made by me or someone else, I just like when mods I like are made and exist. I don't like if someone just copy-pastes one of my mods or does a copy-paste but with the slightest change cos the first of those isn't making anything new and the latter made only a tiny something new while pretending it made much more.

Thank you Dr Jimothy, I really appreciate your answer :)

I hadn’t done anything as of yet because i’m busy with real life right now. Based on your approval I will make a show legacies button. I will give you proper credits and link your workshop. it is the bare minimum I can do

For the flavour pack I’ve been working I will take your expertise and tie it the possible new legacies to a decision.

Thank you again for taking time to answer my questions :)