I have started installing 3/4" hardwood for my customers on a daily
basis.

Since the first and last boards cannot be stapled with the big angular
machines like the Bostitch MIIIFS how do you do this? Manuals I read
say nail to the face of the hardwood strip and countersinking the
nails/screws. While I saw a seasoned installer affix 18 gauge brad
nails because I guess that is much faster. Are there better methods
and what are they?

One method I tried is to put brad nails to the tongue. But when I try
to put 18 ga brads to the tongue the brads only go in properly half
the time, rest of the brads don't go in all the way.

Another question is, what tool to use to cut off the 3/4" vertical gap
at the bottom of door trim so the hardwood strip can squeeze in under
the trim?