There could be a number of causes for the engine issues, nobody on
here should be able to say which one it is without doubt.

Personally, I'd check the records and see when the last time the fuel
filter was replaced. If it has never been replaced (and that can be
too common) then replace it. If it's dirty you're fuel pressure will
drop passed the filter. Since before hitting the throttle you'll have
full pressure it will slowly bleed down the pressure until the engine
starts to choke.

I think some of the other issues are non-issues because they would
affect the boat at all RPM ranges. What's the condition of the
cooling system? How about the engine timing? Is there a vacuum leak
causing the advanced timing needed for high RPM's also bleeding off
retarding the timing causing it to fire prematurely?

It's not to late to get the boat inspected, it could save you a LOT of
money should something go badly wrong. Search for Marine Surveyors in
your area.

The trim tabs are acting as they should. As the flow of water
increases or decreases the amount of lift is altered. Think of a 747
coming in for a landing, in order to maintain enough lift at a slow
speed flaps are deployed which are not needed at higher speeds. The
angle of attack is directly proportional to the fluid movement.

I'm going to assume what you experienced with the steering is normal
due to the forces on the stern drive. From my personal experience
higher speeds do make it much harder to steer, sometimes in fact there
is fear the wheel will slip out of hand and suddenly jolt the boat
into a different direction. As far as I am aware, that model does not
have power steering. I personally don't know of any boats that do,
maybe at sizes beyond my financial abilities which keep me in
ignorance.