Good questions:
- breed a 99% hybrid with a pureblood and get pure children?
Yes, 50/50 chance.
-can mutation give a character the grateful trait or be born with it without breeding with one who has it
No, I made it so the only way to get Grateful was on existing unique characters or by breeding them. Also when breeding for Mental traits, if the offspring is 'child' they won't inherit them, if growth is accelerated to 'teen' there's a chance, and if growth is accelerated to 'adult' they will usually inherit them. Lore-wise, this is a biproduct of how long the sentient mind has to develop during the accelerated growth employed by the dimensional crystal.
All that said, if you want to tweak it so that Grateful is a possibility in either case, you could without much effort. Heck, mutate isn't an implausible way for that to happen. If you'd like I can make that tweak happen and upload.