Lots of questions there. Let me try and help with the MCT part.
Personally I'd be comfortable with a 3 hour connection, even from T3-T5. It's well above the 90 min MCT.

Fare rules. Whatever channel you are booking via should provide these to you. On ba.com they are fairly clearly presented. Be sure to check them for all sectors, as generally speaking if they differ, the most restrictive applies to the whole ticket.