DO NOT hammer the end of the axle. You will break it. Put some good quality lithium grease on the splines, slide it into the gearbox and push till it clicks and the end housing is all the way up against the gearbox casing. It really shouldn't be that hard.

Ideally it should be level. It should hold 2 quarts so as long as 2 quarts are in there, it doesn't matter if it is level or not. Also, i would use Honda MTF. It is more expensive but my manual seems to shift easier since I switched from 10w30.

just my 2cents