One of those would be enough. You can use more, but only in paralell.
No, they are rated for 5v, but they work fine down to about 3.5 volts. Below that blue starts getting weaker, but it still works down to 3 volts, and by that time it’s time to charge the battery.
Yes.
Voltage isn’t one of those “more is better” things.
For voltage, you have to stay within the specified range, which in this case is 3.0 to 5.5 volts.
Battery current is a “more is better” thing. Regardless of how many amps the battery can provide, the board (or any other circuit) will only draw as much as it needs.
Not just in theory, people do it all the time.
Note that not all 18650 are born equal. You need one that can provide at least 10A (15A is better) to be able to provide current to all those batteries. If you add more batteries (in parallel) the only difference will be that the battery will last longer.
You need resistors to protect against shorts, but that’s different from the type of resistors needed to drop voltage in a regular LED setup. For neopixels, each pixel already contains the current limiters needed, so no such resistors are needed.