I am not clear that a fixed term parliament would necessarily prevent the removal of Cameron as PM or prevent the breakup of the coalition. It would prevent the holding of a fresh general election, but it will not prevent the L/Ds from leaving the coalition and joining the rainbow left.

Also, if MPs die or resign (take the Chiltern Hundreds) there will be fresh elections which can make small changes in seats.

A simple majority could undo the legislative changes that purport to require a 55% vote (that's called "Parliamentary sovereignty" and it is fundamental to our constitution, at least according to Dicey).

The really really interesting bit is whether the new Supreme Court would have the power to decide on the validity of a 51% vote purporting to do what Cameron says he will make a 55% vote necessary for.