Are there any guidelines for the "IBM Premier Level Business Partner"?
I do not know if what you are looking for is stated in some place. The closest thing could be PartnerWorld in IBM: https://www-304.ibm.com/partnerworld/wps/servlet/ContentHandler/pw_home_pub_index

I think a partner is just people close to IBM, with a lot of IBM product knowledge, but they can do almost whatever they want with the client, they just have to follow some criteria based of sells, courses, etc. This is the reason, there is a big competence between partners.