Deviated Sects

Q: At our University we have Salaat al-Jumu'a in which 85% of the followers are of the Ahlus Sunnah wa al-Jama'a and 15% are of the Twelver Shi'a. The Shi'a stand in our Saf (rows) and pray with us. They do not bother us, nor do we bother them. But at the end of the day they are Shi'a, so should we the 85% tell them to leave and not pray with us? 

We have overheard their discussions in which many say wrong ahadith/rewayat, but their leader on our campus I have heard many a times scold the Shi'a and say that Hazrat Aisha is our Mother and is to be respected even if she made a mistake. Their leader keeps them in check. So are they Muslim? Can they pray with us?

Q: I am Alhamdulillah married and my wife's father is a qaadiyani. However, I am not too sure what type of qaadiyaani he is. I know he does not believe in the fool Mirza being a prophet but I think he does believe in him being the Mehdi, even this I am not to sure about. If not then he just gives some standing to him. My mother in law is a sunni muslim, how they came about getting married is because they are 1st cousins so I don't think much attention was given, the family is not well linked with Ulamas etc. At first I was not too sure how deep my wife's father was in qaadiyaanism. Initially my thinking was because his siblings were all qadiyanis he had some weakness towards it and just said he is a qaadiyani because from what I know he practices Islam the sunni way and talks very highly of prophet Muhammad (Sallallahu Alayhi Wasallam). But my mother in law came over (they live in Canada, I am in the UK) and I finally got the courage to ask her a few questions about my wife's father and from there I have become skeptical and now I am not too sure how deep his belief is. He does on occasions like Eid pray behind their Imaam but again it is because all his family are there. I was introduced to my wife so I don't know the family too well, however neither did my wife know her father had these type of beliefs as her mother always brought her up in sunni Islam.

Here are my questions:

1. Is he a Kaafir?
2. If my mother in law believes him to be a Muslim does she stay in the fold of Islaam?
3. My In laws are very kind to me so what sort of relationship should I keep with them?
4. Its difficult for me to ignore him out but I am beginning to feel some serious hatred if his belief is kufr, should I still go visit them in Canada?
5. How should my wife's relationship be with her parents?

Q: I would like to find clarity and closure on the school of 'aqidah that is ascribed to Imam Ahmad ibn Hanbal (RA) which is refered to as the Ahl al-Athar or the Atharis. What are the core differences between this school and the other two viz. the Asha'irah and the Maturidis? Is the Athari school that is claimed to be followed different to that followed by Imam ibn Hanbal and other early Hanbali 'ulama? A clear explanation on this issue is required as many are being confused and led astray by bringing up the Athari 'aqidah.