AV regeneration is about 95 HP/sec at level 50. Assuming some Resistance to damage, and covering for misses and such, you want a minimum of 150 DPS to solo one. A full (-100%) Regen debuff on them takes care of 95 of that 150 DPS, and it's also not affected by Damage Resistance. Your assumption is correct, you cannot have negative Regeneration. One problem is that BR's -Regen added with Traps end up superfluous... if you have enough Recharge in Poison Gas Trap to keep it permanent (which is a lot easier than you think... the poison gas lasts for 30 seconds, BUT the debuffs from it last for 10 seconds, which means that its total debuff duration is actually 40 seconds), then the AV doesn't regenerate whether or not you're using Beam Rifle.
Dark Melee puts out a lot more than -50% Damage, because you get:
1: Darkest Night
2: Fluffy's Darkest Night (as good as yours)
3: Twilight Grasp (-10 for 20 seconds, stackable)
4: Fluffy's Twilight Grasp (same, with usually about a 15 second cycle time, so he averages about -12% damage long term)
If you cast Twilight Grasp once in a 10-second cycle, it heals, does "damage" since it stops about 150 HPs from being regenerated during its debuff duration, and averages -20% damage. Total -Damage with the above stacking is about -90% base, before multipliers for the AV's resistance and for -Resist. Note that for Beam Rifle this is a little more difficult since it causes regular re-draw. On the plus side, Twilight Grasp is "adjustable"... if you're taking noticeable damage you can use it more often, and if you're not, you can stick to shooting.
-Resist debuffs do not stack with damage Resistance... if an AV has 50% resistance and you hit it with a -50% resist debuff, the resistance lowers damage (and Damage Debuffs) by 50%, and the -Resist debuff increases damage by 50%. So you end up with Damage = Base * 0.50 * 1.50 = Base *0.75.
I personally also love number-crunching, and in CoH soloing AVs is the main test for number-crunching. Well, there is the Pylon test also, but I find that to be too limited, since the Pylon only does Smash/Lethal damage with no debuffs or controls, so it's not nearly as difficult a defensive test as "soloing most AVs including those with debuffs and controls". Note that "soloing ALL AVs that you meet in solo story arcs" is also possible, but far FAR harder, since there are a few that cause special problems and are very hard to solo.