Battery charge is low.
Whether it is battery itself or lack of recharge, can't say for sure.
However the fact you could start the engine suggests there may be a decent amount of stored energy in the battery and you can probably revive it.
Put it on a current constrolled charger at 2 amps or 5 amps. So a voltage reading after turning the lights on briefly gives a truer indication of energy stored in the battery.
Turning on power briefly removes any surface charge.
Removing the connection on the regulator opens the field circuit. The alternator can not produce power without allowing electicity to flow through the field windings.
So that's all normal.
The alternator at 14.0 an the battery at 12.4 V indicates resistance to flow.
What did the ammeter show?
This is one of the keys to figuring out where the problem lies.