I've been pretty much 'round the bend on this issue, and ended up running Indigo and the server in a separate user space with its own Apple ID. With Fast user switching, the Indigo user space can run in the background, I could switch to my main user space, and the Indigo server would keep running... And I could send out text messages from that 2nd AppleID(just the email address) to my main Apple ID/phone number.
I tried doing what you said, using two user accounts and then fast user switching. Seems like it should have been the ticket, but it still didn't work. I even created a brand new Apple ID just to use with Messages and the new Mac user account and it would not work.
Then I got creative and had a fit technical nostalgia regarding file names etc. The Gmail account I was using had periods separating the three words before the @. I knew from work experience that older operating sytsems and servers used to be very sensitive about "special characters" in file names etc. So I did two things: First I created a even newer Apple ID using my iPhone so it could have an @icloud.com email. Then I created an email with no special characters:
xxxyyyzzz@icloud.comI'll be danged if it didn't work like a charm. So it's either Apple's system doesn't like Gmail addresses with special characters, or it wants an iCloud address - or both. Either way it's working now after more than a year of frustration. I spent the whole day working on this and some other iCloud related cruft. I dream of a day where Apple creates an entirely new system and makes sure these aggravating "features" are eliminated.
PS: on an iPhone if you go to Settings>Mail>Accounts>Add Account>iCloud>Create A New Apple ID this let's you make that Apple ID using (and creating) an iCloud email. You cannot do this on a Mac - it requires you to already have an email address which is why I had been using Gmail accounts. I like this method better since there is only one account to manage instead of two.