Ok, here's what I know bout Invisiblity...
Yes it fails a lot, at lvl 4 and at lvl 20. Get use to it, seems they took
it down quite a few notches early after they went into production. Also,
some creatures can see invso, so invso or not, they'll attack if they see
you as a threat, depending on your fraction and diffculty level. I would
suggest traveling while invso as if it's not on, running or going around
guards/creatures that would normally attack if at all possible. Use your
illusion when you can, then cast invso on yourself, it'll let you stack it
and guards will defend you that are fooled by the illusion.

As for Jewelcraft, here's what I know

Stone Silver Electrum Skill
Jade +5 mana ring ? 30+
malieche(?) non-magical +2 sv posion bracers 35+
Lapis Lazuli non-magical +2 sv disease earings 41+

Do note, the listed skill is not the bottom requirement, it's what I had
when I first tried making the item, so it may be possible to craft them
with lower skill, but not by much Im sure.
I am currently 49 skill and am working up my bank to get back to it. I'll
post more when I have the info.

Telan Ka'Rlaah of Rellos Zeke