Let me challenge your notions for just a second.
First, let me clarify that I don't disagree with your points.
Now, having said that, has anyone ever allowed this combo into a game? I would like to hear from them!
Here's why:
Favor is a 4th level spell (r: touch) with a relatively short duration 1 rd/lvl or 1 min/lvl.
Monstrous Regen is a 5th level spell (r: touch) with a VERY short duration (1 rd/2 lvls).
If you had to blow a fourth and fifth level spell, effectively in combat, how does that compare to other fourth and fifth level spells? Is it really that much tougher? As pointed out there are a number of ways around it...dispel, anti-, dead-magic zones, acid damage, fire damage, and frankly, just because you're not taking damage, doesn't mean you can't be stopped...any will save, insta-kill, etc. Hmmm....
Now, do I agree that an Epic/near-epic character with a persistent monstrous regeneration and persistent favor of ilmater is pretty wickedly tough. Again, on comparison, what could you accomplish with any other 8th and 9th level spell? Also, I'm sure persistent spell has been the topic of some debate on the boards. It certainly has become one since I started using it on my Epic cleric.
I'd love to hear from someone who's seen this in use during a campaign. Worst case scenario, they both serve their different purposes, but subdual damage (immune) must come from a source that was originally subdual damage.