What is your question here exactly?

Sounds like you and/or the SQL service account don't have permission to view the users in the other domain, or the trust between isn't high enough. This'll be a network...

A quick Google and a link to Microsoft docs (specifically look at the 3rd sample T-SQL): https://docs.microsoft.com/en-us/sql/relational-databases/stored-procedures/execute-a-stored-procedure#TsqlProcedure

Without seeming harsh, executing an SP is one of...

Personally, I would use an expression to add the hyphen in then. Assuming your field name is of (n)varchar type, and is always 10 characters in it's original form:
=LEFT(Fields!Matter_Number.Value,...

Where is the value of your file number coming from? Does the source it's coming from contain "xxx0000001" as well?

It is also worth noting that not using the semi colon( ; ) as a terminator has been deprecated. I imagine it is going to be a VERY long time...

c(j) was just an alias. You can use what ever you like for it (for example jobs(job), and then you use jobs.job.value).

Without any sample data and expected result, I'm strugging to really visualise what your asking here. In simple terms, yes, you can join on dates. Depending on the date(time)s you...