>1) Some spells apparently check against con (plague, poison,
>and the like), does this mean that against them it doesn't
>matter how much save vs. spell (svs) you have?

Yes, that's correct.

>2) Does saving vs. neuro disrupt only check save vs. paralysis
> svp), i.e. again it doesn't help you if you have -50svs?

Correct.

>3a) What about spells/communes that do mental damage, does svs
>have any impact or is it only a check against save vs. mental
> svm)? Or a combination of the two, average or something?

Depends on the spell, though yeah I would expect most of them are "mental" saves. These care about save vs. mental, INT, and WIS most of all.

Note that for poison/disease/mental saves, the relevant resistance/vuln is a factor (e.g. resist poison helps you make poison saves) and some skills are a factor. For example, being berserked helps with the body saves (poison/disease) but hurts with mental saves.

>3b) E.g. PWK, which IIRC does mental damage if it doesn't kill
>you: is the kill chance against svs, but then against svm to
>determine if you take full or 50% mental damage?

If I remember correctly, PWK is just a spell save. If you make it, you always take the damage. If you fail it, there is a chance you may die, otherwise you take the damage.

>3c) Generally speaking, is svm mostly just an "anti-bard"
>save? I take it both affects whether a song malediction lands,
>and if it does only 50% damage?

It's the relevant save for most songs, although it's also relevant for some other mind-affecting spells and communes as well. Some of the new shaman path communes fall in this category, for example.

>4) What if you add spellbane to the mix, does it work better
>the more svs you have?

Part of the check to see whether spellbane works or not is making a spell save, so generally things that help you make saves (including -spell) are helpful.