I will recommend that your free versions should be limited, not just changing details but also short, like... take the full game, take the first half or less out of it, and only let the players play that first part for free. If the players can access the full game for free, many won't donate to play the same game. (If you can, use a cliffhanger to 'tempt' or 'push' players into buying the game >:-) )
Web versions will cause problems with both text and images (specially with images) i think this is because the delay from the client's side. But if you wanna do that thing of the text box out of aesthetic, cool.
Something like this? I think is doable in RenPy if you use the textbox like character's sprites. But i am not sure, never used that engine. :-|
