This is a twofer, of sorts:

1. Specifically, how does the skill affect regeneration?

2. Do items with "Regenerate HP/Mana over time " stack?

Ffff~

...ok, that's wasted money and skill points then. Yay.

Related: How much do these items regenerate by themselves and does that stack with the skill (for HP)?

items with regen effect give you 1 HP/SP per 5 min (in-game). They stack with skill (item + grand Regen = 5 HP / 5min) but not with each other.

Basically, only mana regen is worth it. Particularly for your cleric since the heal spell is only 2 mana.

Just ignore regen and get all your casters 1 form of mana regen.

(also, grand master water magic and you can enchant items without enchantment if you're having trouble finding a mana regen item worthwhile)

Regeneration spell stacks both with skill and the item. The amount <-> skill level is the same (2, 3, 4HP per 5 min at grandmaster). Higher Skill value gives you longer spell duration.

Grand Regen skill + grand Regen spell + item = 9 HP / 5min

Mhm, that sounds pretty solid. :)

In MM7, the regeneration spell is ridiculously overpowered.

It's 5 / 15 / 50 HP per 5 minute!!

I had to ban myself from using it. Good thing they toned it down in MM8

I never really used regen when MM7 came out. Then, after a decade of MMOs, when I came back to the series I was floored at how useful it was.

In MM8, regen is very helpful in the early levels.

Talking about "regeneration" of sorts... i'm not happy that regions apparently repopulate after a certain amount of time. I clear these things out for a reason. :|

You're out of luck, it's true for all M&M VI+ games.
Depending on region, respawn takes from 6 months to 2 years.