Your probably correct. Not many carriers get AC then change the engine type. Ususally because there have been many other examples available either new or on lease. However, if the AC is relatively rare and commanding high lease rates, then I guess some may investigate the possibility of doing the above. Its all relative though, as the same airframe can have differing values due to cabin fit/engine type/avioincs fit etc. With market forces at work it is partially dependant upon the airlines' attitude to financial risk as to the path they take.
DTV the cabin is far more complex with the modern IFE systems and galleys now being used. So if operators have 2 main galleys and a 3 class layout it will obvioulsy be of little need to a one class/ charter operator. bmi along with lots of other operators have a set service plan. Its much easier to have a common cabin layout, so trying to achieve commonality with used AC is harder than new airframes built to customer requirements. An engine change is completed in a day a cabin config change with ife systems galley changes with all the services modded and a new installation plus the re-weigh starts at weeks not days , all with the obvious cost implications.
6 If bmi bmi took A330's with Ge motors then they would have problems with etops. If the GE's were operated on non etops routes it may help. But the engineers usually get approvals for the ac type with a specified engine. They can gain approvals for other engine types if the company so wishes. If bmi used a third party engineering down route they usually provide cover based on the AC spec bmi gives them. So if they have no approvals then they can't sign off items, so less attractive to bmi. Cost to bmi would be if they needed to send engineers on courses, but this is small beer compared to securing an AC and the extra revenue it brings. This is balanced however by the actual sense of operating mixed types. Whole company is now single fleet (big savings). As for your douwnroute scenari, its much better to get a company with approvals, so not really an extra cost to bmi, only for the 3rd party if they need to get approvals to secure a contract with bmi.
An engineer can give you a more concise answer!