I'd say 13.8V at warm engine is OK.
Right after the cold startup I'd expect around 14.0 to 14.2 Volts at the battery.
I agree, the voltage between D+ and B+ terminals should be near equal when
the alternator is charging.
When you wrote the regulators are not exactly the same, you mean the new
regulator is not a direct replacement of the old one?
Regarding the bulb question: I think the higher wattage bulb won't affect the
output voltage significantly after the alternator's output voltage reached the
self excitation threshold. And I guess I have to revise the very end of the
quoted statement after studying the mechanism of the excitation in the last
two months. Anyway if you want, you can try the 3W bulb, it won't do any harm.
To rule out wiring problem between the alternator and the battery the voltage
drop should be checked between one of the two B+ terminals of the alternator
and the battery + terminal. It's not a simple job, because the alternator connector
is well hidden under the intake maifold. The voltage drop should less than 0.5V when
"nothing" is on and the drop should be less than 1V when the "heavy consumers"
(rear window demister+blower+low beams) are on.