Hi Kumar,
I would have asked you to check the BADIs mentioned in the wiki - Team purchasing - Supplier Relationship Management - SCN Wiki , but it appears that you have already gone through them.
Just my 2 dime - For a Team shopping cart, can we find all the substitutes of a user who created the given SC (GET_TEAM_MEMBERS) and if the approver who is trying to open the cart, is part of that list of substitutes, then can we disable the 'TAKE OVER' button (TAKE_OVER_ALLOWED).
Hope this helps 
Cheers,
Yateesh