First, a shout out to Vanessa Bruwer @VanessaBruwer and Tyson Paul for their help!
Feel like I was pounding rocks, and had a great find! 
...How to add UNIX agents manually via command line
Required
1. Unix Agent action account and agent maintenance account ID and passwords
2. Unix Resource Pool name (use get-SCOMResourcePool)
Don't confuse the WSMAN login and use your MSAA ID
BTW, cmdlets exist with 2012R2 and 2016
From MS running PowerShell as admin
$MyPool = Get-SCOMResourcePool “UNIX/Linux Monitoring Resource Pool”
$SSHCredential = Get-SCXSSHCredential -UserName scom -ElevationType sudo
$WSCredential = Get-Credential scom
# Using MSAA account this fails
$DiscResult = Invoke-SCXDiscovery -Name "ubuntu.testlab.net" -ResourcePool $MyPool -WSManCredential $WSCredential -SSHCredential $SSHCredential
# Alternative Discovery for Network IP range
$DiscResult = Invoke-SCXDiscovery -IPRange 192.168.1.50,192.168.1.75 -ResourcePool $MyPool -WSManCredential $WSCredential -SSHCredential $SSHCredential
$DiscResult |fl -property *
$installResult = Install-SCXAgent -DiscoveryResult $DiscResult -Verbose
$installResult | fl -property *
Using MSAA account this fails
Using SCOM Agent Maintenance Account
Console verified
Resources
Old https://blogs.msdn.microsoft.com/scxplat/2009/12/11/cross-platform-powershell-scripts-released/
2012R2 (tested on my 2016 lab) https://operatingquadrant.com/2012/12/06/using-powershell-for-automated-unixlinux-agent-discovery/
PoSH cmdlet reference https://technet.microsoft.com/en-us/library/hh545212(v=sc.20).aspx