Few comments on your comments:

The first point is accurate. At no point ever have I seen more than two enemies on the screen at the same time.

For your second point, I've had the same thing happen but with chains much longer. I do believe there is some mechanic that can be exploited. Something with the way the game spawns enemies.

I have noticed the flashing of the guns, but like you said, I think this is just due to the rendering of the sprites.

Emulators do have input delay, with some being more noticeable than others, but an NES on a CRT tv will have no input delay.