1. I use t-mobile myself in the UK with a pay as you go SIM in an N900, so yes.

2. I'm not sure, but I think you get prompted when first connecting to a cellular connection whether or not you automatically do that in the future. So I don't think it'd happen without your say-so.

3. Yep. Don't see why not!

BTW. In the UK you can text 6MONTHWEB to 441 and, for 20 quid, get 6 months internet access, capped at 500MB per month on pay-as-you-go IIRC.

Might be worth just giving T-mobile a call. Would've thought you could get 'unlimited' data bundled with your current plan if you threatened to leave or something.