I've seen some conflicting information on this.

What I know for sure - higher intelligence means longer durations on maledictions and more damage on damage spells.

What I don't know - does intelligence affect the likelihood of landing spells in the first place?

Basically, int and wis boost the AP's spell level. This can mean more damage, great chance of success, and more duration, if for that particular spell, level matters for those things.

i.e. will a 36 fire a-p be casting 36 spells, and a human at greater than 36 or is the baseline at 20 int, with lower int races being penalised?

This may be asking too much but is a 1-point difference always going to affect spell level? i.e. will a half elf be casting at all better than a human or is the difference too small to show up?

No, it's not, if you're less than 19 it may drop your spell level.

In general, yes, 1 point will make a difference, but not always.

It's why it might seem easier to sleep the lower int races. Checks your int against theirs, then saves, I believe?

That's just how it's always worked in my experience. Giants used to always be bait to my Necros (Unless they were ragers, damn spellbane.)

Mental stats provide a modifier to spell saves.