Purchase requistion release problems with no release strategies set

Dear all,

I am involved in purchase requisition workflow development without configuring release strategies.We cannot set release strategies as there are more than 4200 approvers depending on plant and the value of purchase requisition.So,I have used rule to pick the right agent.

Though this workflow behaves as expected,I do face following 3 problems.

Those are when the purchase requisition is under approval it should not be changed at ME52N or released at ME54N((we use custom field as release indicator) or deleted at MEMASSRQ

Please guide me how to over ride the above problems for which I will be grateful.

Thanks and regards,

S.Suresh