G,
Your so so easy to get a rise from !
To answer your questions ....
MTBF = (flying hours * aircraft quantity)/(number of confirmed failures)
More flying hours you do, the more engines you have, the bigger the number ontop gets, MTBF increases.
The silly theoretical scenario question...
Mathematically their would be no chance of an engine failure, as giving your start conditions, their is no failure record, so there can be no mean time
between failures, and no hours flown, the MTBF is zero, and last time I did maths anything multiplied by zero is still zero.
You were right it was a silly theoretical scenario...
Edit : made an error, fixed it