Bitcoin Protocol OP_RETURN + Omni USDT Tether + CoPay Multisig Help
I have solved my problem with a lot of debugging and help from the community on IRC irc.freenode.net #bitcoin-dev and also from people on the bitcointalk thread I started about this Topic.

I invite anyone having similar Copay Multisig Omni Colored Coins Recovery Issues to have a look at the bitcointalk thread here: https://bitcointalk.org/index.php?topic=2500531

The Problem is hereby solved, Details about how Copay derives keys can be found in the linked thread as well as more detail about what was implemented to apply the recovery (command line tool "omni:copay-recovery" on Github).