Your command line doesn't look quite right. Have you installed the executable into your own home folder?

~/thunderbird/thunderbird -P -no-remote

should normally be something like :

 thunderbird -profile <path> -no-remote

You can use:

which thunderbird 

to find the "official" location.

IIRC the -P switch as shown will start the profile manager. You can instead pass either the name of the required profile or its pathname. Check all the variants of this switch.

http://kb.mozillazine.org/Thunderbird_Command_Line_Arguments

1) Is it safe to open multiple instances? I don't know of any danger apart from risks to user sanity. I get by with all my accounts, private and work, in one instance if Thunderbird.

2) Will this feature be deprecated? I doubt it. I don't know of any specific plans to kill it.

3) It is possible to do this on a Macintosh (10.12 and above)? If so, how does one launch multiple instances? I don't use a mac but the command line feature works on both Linux and Windows. Your main problem may be discovering where to edit your launcher.

Thank you very much Zenos.

My objective is to have different email addresses for each of my projects and each project assigned to a different workspace. That way I can flip from one workspace to another to switch projects without having everything intermingled.

Does that seem sensible?

Not to me. If i'm working on Project X and a message comes in for Project Y, I'd like to be able to see the Project Y notification immediately. If you are able to set up strict isolation between your "workspaces" that may not happen.

If you took it a further step and created a distinct user account for each project then you'd automatically get a discrete Thunderbird profile in each user login. It comes down to how separated you want your "workspaces" to be.