I haven't read up on strategies, but from what I've been noticing myself after a few weeks of play are the following.

1. Only 2 enemies can be spawned at the same time in any room. I can't verify this for sure, but I'm sure somewhere in a strategy guide, this is mentioned.

2. There seems to be some random element to which the enemies are spawned. I had a few funs where I had 4+ critters spawn all in nearly the same x coordinate on the screen.

3. The player's gun sprite flashes at different speeds and different colors. On the surface, this appears to only be an artifact of the game engine's sprite rendering logic, but may be tied to something else non-trivial. It appears to have some significance in relation to enemy spawns. Still need to do some research on this.

My suspicion is that the emulator may even lag behind a little bit (maybe not noticeably) but I haven't been able to prove this. It may be best to have the PC set up as a streaming station with a video feed in from the NES hardware. I also assume this has been talked about before at length in the general speedrun forums/discord, but again I haven't spent time looking for comments on this which is my next task.

I have a few other observations that I'll post in a follow-up.

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.

Thanks for your reply.

One of the other idiosyncrasies of the engine I noticed is that the bullets originates from behind the gun, toward the player character's chest area.

Also if you jump and hit a wall and fall, there's no hit-stun like there is when you bump into an enemy and fall, so you can press A to stand up immediately after the fall animation triggers.