Hi Elizabeth
Welcome to SCN!
Your query should have been posted in the Process Integration (PI) & SOA Middleware space as it is more relevant there than in ABAP Connectivity.
First of all, if you are already using IDoc as your inbound interface, you do not need to generate any proxies for it. The reason you are getting the "No implementing class" in ECC SXMB_MONI is because your target interface is an IDoc but you are using an XI/proxy channel to send it to ECC.
You should switch back to the original IDoc receiver channel.
You are getting the "No standard agreement found" when you used the IDoc channel because some part of your configuration is incorrect. I suspect that the target interface in the operation mapping used in Interface Determination does not match the receiver interface in your receiver agreement.
I'd suggest you double check each of your configuration steps. You can also try the Test Configuration functionality to assist you further.
If you are still unable to resolve this. I'd suggest you post screenshots of your Receiver Determination, Interface Determination and Receiver Agreement config.
Rgds
Eng Swe