For 2 most are listed at sunrays dominions site in section what is not in the spell manual .
1) I would like to know also how many commanders in total can exist .
I had at least 300 Commanders in my capitol in my last game though so the number of commanders per province is probably quite high .
The unitviewmap has even about 1100 commanders in one province and everything seems to work normal also .
4) Sickle who's crop is pain is cool if you have an ally who does 1 call of wind on your province with sickle + staff of storms every turn .
This gives you 21 deathgems for 5 airgems each turn . Not a bad bargain
Alchemists stone , Barrier , Aegis , Magebane , Summit and hammer of the forge lord are very good also . Those i always try to get if possible .
Finally the +3 death staff is good too and the area petrification sword and gift of kugri .
Most other artifacts i rather rarely forge .