MPL to CPL/IR
Once you pass 1500 hours you get a full ATPL just like any other. No multi crew restriction.
The only time you would have to do tests to convert to an fATPL is like you mentioned, if training collapses. But if you are doing an MPL, you have been chosen by an airline. As long as the airline is a good one (whole other topic) then you have a job and have no reason to worry about fATPL.