Negative Ghost Rider on both questions. Luck on your suit will help you find it., high mining as well.
Dungeon Mining
For dungeon mining, once an 8x8 area has been checked for saltpeter, it won't be checked again for 5 minutes. In other words, the same location can be checked for niter deposits every 5 minutes, regardless of the success of the previous try.
GM Miners will discover a niter deposit inside a dungeon on their first strike, provided that there is one within an 8x8 tile location.
Ilshenar is considered a dungeon so it is a great place to look just walking down the mountain sides.