Single Button Power Switch and Mode Selector
Your thinking seems mostly ok.

Remember that the self discharge rate of the Li-Ion cell is about 10%/month so this will dominate your current calculation. It may be the equivalent of 300-500 nA continuous current, so battling to get the current ever lower may not help you much in terms of battery life.
You may find this discussion helpful.

If your are using a Pchan switch to power your WS2812b's make sure you set the output port driving it to an input with a '1' in the output register before deep sleeping. You should do the same with the pin driving the WS2812b data line.

There is nothing wrong with sleeping and waiting for a pin change interrupt, just remember to measure your battery voltage when you power back up, since the WS2812b won't work below 3.5V so you need to detect this critical threshold.