If the SA mode will not work by hand, chances are it will NOT work in cycling.
If the reason to adapt the P99 slide to the P990 frame is to change caliber, there may be more work involved.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)