FastLed (WS2812B) and IRRemote problem
The WS2812B chips are highly timing-dependent. You have to clock out bits within a range of ±150 ns. This isn't likely to work well with clocking in bits from an IR remote. You are finding errors on the IR codes - that doesn't surprise me.

I suggest you use your idea of two Nanos. One can be reading IR codes. The other can display a stream of pixels and then "ask" the other one (eg. via serial or I2C) if a different command has arrived. That way the timing can be shifted in such a way that both processors are not interrupted when they are busy.