Indy, the prop spinning in neutral is just the friction in the trans and it delivers no real power. Don't worry about it. Neutral is illusive in these transitions.
The trans should not slip in forward gear! You should feel a definite "CLICK" as it locks into the detent that "holds" it in gear. If it slips it is a simple adjustment.
There is no means to lock the trans in reverse, you must hold pressure against the shifter to keep reverse engaged.
Dave Neptune