As for code, confirm you're aiming solely for N900/Maemo 5 development? I'm no dev but if you aren't familiar with any of the 3 languages you mentioned, from all I've seen here I'd say hit Python first. (I'm dipping my toes in the Python swamp water little by little)

If you know C++ and just want to kick something out fast, then do that. Over time you'd definitely want to head the Qt direction. Both Python and Qt have significant overhead which may reduce your user/test base on the N8xx devices, Python less so. But if you go straight for Qt you're assured N900 functionality and minimal tweaking for future Harmattan devices.

As for Flash 10.x support, considering Adobe's either making deals or handing out Flash 10 passes to all the major players, there's no reason to not expect that down the road. It just doesn't make sense to not get that update for the N900 when Nokia's competition is likely to get it.

You mentioned java based games off the internet, but then later speak of Flash. Java is not natively supported on the N900 and has been painfully and I think rarely made to work on the N8xx but don't quote me (and don't hit me if I'm wrong!) so I suggest using Search here on the site and see what comes up in answer to your questions.

I was under the impression you could playback .swf files in some regard on the N8xx devices already, assuming version requirements are met. I believe either an app to capture the file or steps to do so are in one of the threads here. Again, search, search, then search again.