When Biters switch to Revenge, is the attack intensity relative to how many biters you recently killed? I know the probability is based on the amount, but what about the "quantity"?

Quantity depends on evolution, but no more than 60 per squad (with default settings).
And, of course, if there are fewer biters in the "source of revenge" area, then fewer

source of revenge is any biter nest within range of my installations or pollution cloud, or is it limited to the place where I murdered some?
Very interesting behavior in any case.

source of revenge is random area within a certain radius of the murder site

ah, murder site, ok good to know. Given the continent sized borders I have, it is good to know that butchering biters on one end will not be the downfall of several weak/advanced outposts on the other end...

In other areas there will also be increased activity, but this will not be revenge, but spending "points" received for destroying nests.
Depending on the current AI mode, these can be either attacks or, for example, mass settlement under walls in siege mode

Ah, so tangential answer here to my other question here, you are saying the AI can spend points acquired (because I destroyed nests in some places) in other places far from my destructions. So points are global and not regional.

yes
In general, when attacking a biter, you must be prepared for active hostilities.
It can be assumed that the player himself determines the time when the big war will begin