I keep getting different answers from different people so I thought I would ask you guys to get even more different answers

Going to try to keep the numbers easy in my example, so lets say we have 10 #12 THHN current carrying conductors in a conduit 25 inches long all protected by single pole 20 amp breakers.

So we start out in table 310.16 90 degree column since we are doing deration. #12 THHN at 90 degrees is 30 Amps.

Now from table 310.15(B)(2)(a), we must apply a 50% deration, so that drops us down to 15 Amps allowed.

Now for my question, does this mean that we need to have an over current protection not exceeding that, or only load that is not to exceed that?

Say for example we have a lighting circuit with a load of 5 amps, is this ok with at 20 amp breaker?

Another case, say we have 2 15A convenience outlets on one circuit, this one to me seems that it shouldn't be ok since someone could exceed the 15 amps for deration, but it would not trip the breaker.

The case that seems pretty clear to me that’s not allowed is a dedicated 20A outlet on a 20A circuit. Since it would be easy to exceed the deration?