A company approaches you -the developers- to buy and use (with or without you in the development team) Construct, resulting in a non-free Construct.
Is this situation realistic/what the developers would choose for?
I don't think this is actually a realistic situation. For a commercial company to buy out a GPL program like Construct is pretty tricky: the source code and binaries are currently freely available on the internet, so if a company commercially adopted Construct, there's no immediate incentive for any users to pay until enough work has been done to improve it to make purchase worthwhile. This is probably too big a barrier for any companies to overcome (except the really big guys, who wouldn't bat an eyelid at a project this size).
Still, if it did happen, personally I would insist us developers are still on the team. Things like the object picking algorithms that power the event engine are so complex I doubt any other developers would be able to sensibly expand on it without our help.
Currently, their whole business model is based on donations I guess
We're not a business, hence we don't have a business model! Donations are there simply to support the project/developers, it's not part of our "business". We haven't got any plans right now to become a business either. While in theory we could go commercial, we face the same problems as above (overcoming the freely available branch) as well as being obliged to provide customer support, regular updates, put up with pissed off users who want their particular bug fixed, etc etc etc. So right now we haven't got any plans to go commercial ever. Free for now!