Re: 2 players, the first movie I verified was a 2 player run:
[1556] NES Rush'n Attack "2 players" by Randil in 09:35.90 My old device could do that. Right now the only things this new device does that my old one doesn't do are play Genesis and support high-bitrate playback. Oh, and it doesn't require a computer to play back and is easier to set up, and the connectors don't break because you aren't soldering Nintendo's frail, unsolderable wire. I even made an adapter for the old device so I wouldn't have to deal with that (see ~3 posts back).
Re: Kirby, unlike most NES games which poll only when they can use input, Kirby polls every frame, even if it lags. Now this wouldn't be an issue except for a second property, which is that timing in console is different for Kirby than it is on emulator, so it desyncs. The emulator seems slightly fast compared to console. With other games this very well may be happening at times but it is masked by not asking for input unless it can use it.
For this specific run, had you read the thread, I already tried it.
See here for details.
Basically emulation needs to improve to verify Kirby, and when that happens the runs will need to be re-run on the new emulator.