I think you have a correct understanding. Once you get a NF, ref "holds"..
the points until your opponent is out of immediate danger. Usually, that's when he gets back on his belly. Once that happens, you can score again, with the same hold or a different hold. If the ref is still holding your nearfall points, you can't score more while your opponent is still in danger. (I expect that if the ref is holding a 2-point nearfall and you get a four count while he's still in danger, you'll be upgraded to a 4-point nearfall.)
The end of the 2nd period at 141 was a good example. Brent had Longo on his back for the last 30 seconds or so and the ref was holding 4 the entire time. The 4 was awarded at the end of the period. I wasn't paying full attention and was wondering if Longo had gotten to his belly and they forgot to put up a third NF4 when Moore nearly had him pinned at the end. But I watched the replay and was clearly wrong about that.