Is it a SQL authenticated user or a Windows Authenticated user?

If Windows, create the login on Principal and Mirror.  Map the user on Principal to the database

If SQL, create the...

Add the user account on both sides of the mirror/AG ensure correct SID mapping etc, re-snap the database.

As you say permissions are inherited from the original database so your user...

Did you do the install via "Install-Module" or was it the offline install mode?  If offline have you unblocked the child items and imported the modules first off

Import-Module dbatools

Copy-DbaDbDataTable

Visual Studio has a schema compare option if you have the database tools installed.  It's free but can be a bit clunky as it pulls in differences you tell it...

You will need downtime to move the VNN, your probably best to create a DNS alias instead using a CNAME for when you do the actual switch.

Create the new cluster,...

Also are you deploying Standard Edition or Enterprise Edition in this new setup?

As if it's Standard you have other complications to contend with given how it all needs to work.

A traditional clustered instance, along with a node for an AG can co-exist in the same cluster, but there are caveats to it.

Primarily is the fact that the AG replica...

Never heard of it but from a quick google it seems its another DELL EMC product.

We did have Data Domain at one place I worked another DELL EMC product which...

I use ASG Remote Desktop the freeware version is great, ok it's limited as you cant have more then 3 RDP session open at anyone time, but I find it a...

unsure, it depends, you may break it more than its already broken.  its a risk you will need to take hence why I would build a new server and move...

do they have read access to master?

we had this recently on a 2012 box and changing user permissions to be ssis_admin, they couldn't run the ssis reports without getting errors...

I would be cautious in this, if something has borked the installed on the engine, then installing SP2 may resolve it.  Don't 100% expect it to work though.

If it didn't...

Remember that 2008R2 and below are all now out of mainstream support and extended support.

SQL 2012 and SQL 2014 are now out of mainstream support.

These legacy versions of the engine...

Compatibility level doesn't work like that.

If you on a 2017 server you get the 2017 syntax.

Changing the compatibility level is only for some internal things to revert to the old...

post the full details of the job output, not just what the overall job says, theres more to the job, expand it and select the details from the step