Yes, USCIS will issue an RFE for the divorce decree if you file the waiver case without one. Hopefully, you will have the decree in hand by the time you need to respond. Not sure of the timeline USCIS will ask for it so I would make sure the divorce is fast tacked. I751 cases are slow so hopefully it will some time before you get the RFE.
Also, when you file the i751 waver you will need to provide good faith proof of the marriage for the time between the time you were married to the time of separation. So, you will need proof of cohabitation and commingling of finances. Joint lease, joint taxes, joint bills, photos, affidavits, joint bank statements, joint insurance and so on during the marriage when you were together.