Oh boy....
1. There are two different Endomag versions. One that is made for the CMMG RDB that relies on the ejector in the bolt. The other one has the ejector built into the Endomag on the mag itself. I do not like the latter since you must leave the mag in the gun to clear a live round.
All that said, they both are designed for different feed heights within the Endomag design. The CMMG RDB uses a barrel extension like you would see on a standard gas operated 556 barrel and requires basically the same feed height as a 556 mag. With a straight blowback 'Colt' 9mm design, the feed height is higher. On the OLY system which is also straight blowback, I do recall it uses a fixed bolt in a carrier but I can't recall what the feed height needs to be.
Regardless, I haven't heard of anyone using an Endomag with an OLY system but sounds like you don't even want to use the OLY system and just salvage the suppressor for use with your CMMG right?
2. I'm not clear as to what you have going on with that second to the last picture where you have male threads coming out of the suppressor. Does your barrel have female threads and the can screws in that way? This isn't an integrally suppressed setup is it? Maybe if you posted a picture of the entire setup it would help.
3. I think this is wasted effort to use that can. Many great cans out there and I wouldn't bother or just plan to get the can modified to work with the CMMG RDB. I would send the suppressor to Curtis Tactical and have him make the can user serviceable if it isn't already and re-do the mounting system depending on how it is now as I can't tell from your pictures how it is mounted.
I have an integrally suppressed 9mm setup using a ported barrel for the Colt system and I've been wanting to get it re-cored and then have a custom SS CMMG barrel made to work with it but am on the fence to doing it in hopes that MEAN Arms releases their delayed blowback system which I hope will be superior to the CMMG system. Then get a barrel turned down for that system because both the CMMG RDB and MEAN have extensions where the bolt engage to create the delay unlike regular straight blowback systems.
You can see pics on my site of what Curtis Tactical has done for me and I've always been very impressed.
https://c3junkie.com/?page_id=244
https://c3junkie.com/?page_id=553