I was following the achievement guides in this section and accidentally got myself locked out of at least one achievement because of lack of specifics and the game having saved my conversation progress (automatically, with only one save slot). Instead of having to reset the entire game, I found a fairly easy way to manipulate the saved data for my purposes, and hopefully this might help some other people as well.

The files containing saved progress can be found in this folder:

...\Steam\userdata\<user id>\717610\remote

Specifically, the (XML) file named SavedPassages can be opened in any text editor, and by removing the strings corresponding to a given conversation choice, you can reset parts of progression.

For example, to fix my accidental issue locking myself out of Ella's achievement, I removed the following strings related to presenting her the Newspaper, restarted the game and was able to reset that subject so I could get the achievement without having to start the entire game over:

<string>BilliardsRoom0_JazzSingerNewspaper</string>
<string>BilliardsRoom0_JazzSingerNewspaperDrop</string>

Of course, you will have to figure out yourself which lines actually control what. Obviously you will want to backup the file beforehand in case you mess up your save.

This has been tested with Steam Cloud enabled only, however I did not need to change any Cloud settings in order to get this workaround to work.

Hope this tip was of some use to you, and let me know if anything was unclear or you have general constructive criticism/feedback!