How do you thin Bondo Lightweight body filler?
I think that the answers to your questions should be no and no. Because these filler products all tend to set so dang fast, there may not be enough time for the alchohol to evaporate properly. Mixing less hardener is not going to necessarily mean a slower set. It is more likely that you will get an incomplete cure - the kind of sticky mess that happens from time to time.
BUT is there anything wrong with trying it on some scrap first. It would pay to check cure, adhesion and workability.
Thinking about this on the run as it were, if the "repair" is a largish area but very thin, it might be better to "paint" multiple layers of epoxy resin rather than use a filler. It will take longer, might cost more but should give a far more durable finish.