So long as the domain is toxic and the toxic spirit is within it, the Background Count increases the spirit's Force. If the domain is not toxic but simply has a Background Count, then the toxic spirit does not get the boost. Also it is not immune to the effects of very high Background Count ie manawarps.

Not precicely. There is nothing explicitly stated either way as to what happens to a toxic spirit when the background count goes over 5. If it is caused by a toxic effect, then there's evidence that the spirit may be immune to it, since it's caused by its own domain. The empahsis is on "may", though-- most spirits are fully immune to the effects of their own domain-- a Spirit of Flame, for example, is completely immune to the volcano it's summoned from. Still, this isn't a set rule, just a standard special effect. However, since the spirit would get a power boost, I'd think that a low-grade manawarp would act just like a normal background count for the domain's toxic spirit.

However, Horrors are another kettle of fish entirely. Horrors cannot enter our realm unless the mana level is high enough; but they can *survive* in low-mana areas practically indefinitely-- it depends on the Horror, but this much is a clearly stated fact in Harlequin's Back, among many other sources. The exact level of mana required varies from Horror to Horror; but there's even a suggestion that certain Horrors can function just fine during the lowpoints of the downcycle-- Aina's son, despite being half-Horror, wasn't discomfited at all. In fact, apparently he had access to some magic, and was fully capable of teaching a Great Ritual to people-- and it came as a great surprise to him that it didn't work for humans.

Other than the fact that they require a certain amount of mana to cross over, and a much lower level of mana to survive, there's nothing suggesting that the Horror's realm has any given level of mana. For all we know, the Horror's realm could be one big manawarp-- they don't require normal Astral Space to survive, after all.

While that might be true for that specific Domain, I don't recall anything in canon about Spirits or Elementals being immune to their own element. In fact, I believe it states that they are more vulnerable to Elemental damage, and does not give any exceptions.