If you lose access to your saves there's no way to get them back, I'm afraid. Ren'py doesn't support cloud saves (and Google won't allow the use of their cloud services).
As for starting at Act Three, this has been asked and answered multiple times so I'll just re-iterate the main point here: there's a reason why no sandbox game in existence, including AAA games, allows you to do this. The sheer amount of variables involved mean that it's a Sisyphean task for any programmer, and any player would need to spend at least thirty minutes going through a series of "Did you do X, Y, or Z? How many times did you do A? If you did X, did you also do B or not?"
You can see why, as a solo dev, I won't touch that mess with a ten foot pole :P