I've never heard the terminology your using for the moves before but i guess i'll try to explain here...
I suppose by the L-thing u mean: when falling from a blow towards the ground, L can be pressed right as your body hits the ground. it takes a little practice to master; it was easier to do with the Z-button on SSB. You do a stationary flip after pulling it off or you can tilt the control stick to a certain direction as u press L to immediately roll to that direction. Pretty useful for quick recovery (i might've misinterpreted your explanation, but i hope this helps in some way).
I think i know what you mean by wavedash. I've done it sometimes, usually by accident, then kinda figured it out like you did with jumping in the air then doing it as soon as the ground is hit. And I'm pretty positive it cant be pulled off at a stationary point. I think roll dodging's supposed to somewhat serve that purpose.