As to the 12th power energy field, I think the point is it's a gigantic/shocking/terrifying level of energy. Albeit not canon, Roddenberry's novelization indicates that the "cloud" is the glow of annihilated atoms hitting this powerfield. The cloud itself may be mostly harmless, but the very fact of its existence points out that whatever is making it is probably not something to be trifled with...going back to the "stupid Klingons" question.
You see i've always assumed that the "12th Power Energy" thing was essentially whatever the maximum output of the Ent was to the power of 12
Spock says "The Cloud is creating an energy field, greater than the Radiation of Earth's Sun" which suggests that 12th Power Energy is somewhere in that ball park
One thing that I still find fascinating is Kirk's order to self destruct (not included in all versions), what was he thinking exactly? The Ent blowing up would have very little effect, although they were in close proximity to V'Ger itself, whats to stopping V'Ger from enveloping the Ent with a forcefield and containing the explosion?
It was probably him feeling lucky. Antimatter explosion seems to be the go-to weapon when dealing with giant things like space amoebas, etc, so blowing the Enterprise up while within V'ger seemed like a reasonable gamble. Also, TOS has always treated antimatter as much more powerful than what we understand it to be today