ALT-N-N-M-L-L should toggle between the two modes.
It's ALT-N-M-L-L on other BlackBerrys, but since the N key is also the B key on the 7100t, you have to hit it twice.
Don't know specifics about the RF level. I know that RIM folks I've worked with don't like to troubleshoot on devices that that are getting 100-256.
111 is the lowest I've seen my 7100t go before it loses signal completely (256).
My house is in a very weak T-Mobile signal area, and my 7100t usually shows between 105-110. For as low as that is, it does a pretty good job of making calls. I've had NOKIA phones that do a lot better, but that's what they're known for.