I'm not 100% genned up on the US plans but as no one else has replied I'll see if i can point you in the right direction.
1. If you disconnect your data plan you won't be able to connect to blackberry via WiFi. You will have the internet browser for WiFi and that's it.
In the UK they offer pay as you use data plans, if you can get something like that then you would have access over WiFi and as the data wouldn't be going over the carriers network you wouldn't be charged.
2.The WiFi has priority over the carrier network so if you connect over WiFi then you won't be using tmobiles internet. You can also turn off data services under Options->Moobile Network.
3.Opera Mini doesn't currently support using WiFi, there has been mixed sucess in getting around this.
4.AIM would be using data, more specifically blackberry data so you will need the data plan even to connect and use the WiFi.
So the key answer for you is that you will always have free WiFi to use in the browser but to use any IM's or email, etc you will need to pay for a data plan.
Once you have a data plan you get the blackberry data over WiFi and this data usage is free.
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BlackBerry Bold2 9700 / 6.0.0.448 / BESx
Blackberry Playbook 32gb WiFi / 1.0.6.2390
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