Oh Veldy...you're a man after my own heart.
1. One of the arch lectors of Sigmar resides in Nuln which is still 'attached' to the province. Wissenland was the site of a miracle of Sigmar, where he/his spirit/something saved a group near the mountains. The tribe that were the founders of Wissenland were staunch followers of Sigmar during the founding days. Sigmar's Heirs (sourcebook which is very good) stated that Taal and Rhya were quite popular in Wissenland as well. Myrmidia's influence is more prevelant in Averland, but extends to Wissenland somewhat as well as there's some influence from Tilea due to the various passes and proximity. Wissenlanders are known as very pious people, and somewhat superstitious as well, and are known for their devotion to many gods, though, too. So they make it a point to make their rounds to all the different deities through the week.
2. Wissenland is a poor province and relies on more basic troop types and hit and run tactics. Horse archers are a mainstay as are pike regiments from Wissenburg (could be used as spearmen though). The bow is a common weapon as well and using huntsmen and archers is quite fluffy. Anything that's expensive wouldn't be that common. A few regiments of swords or greatswords or cannons would be alright (especially if there were connections to Nuln from one of the nobles).
3. Not many big knightly orders really. The order of the Broken Sword I believe is one. And that's the only one that's coming to mind at the moment. I think there is another, but my brain isn't working at the moment. There is a listing of different knightly orders in the library and it's quite thorough. I'll have to do a bit of research and refresh my memory and get back to you.
4. According to 'A is for Altdorf' by rufus sparkfire (a recommended read for sure, can be found in the library as well) Beasts and Grey wizards are the most common in Wissenland...but by no means are they exclusive.
Hope that helps some. Feel free to ask anything else if needed.