The sample provided follows standard definition for usage from mainframe style extract files - or PC Cobol files that follow old conventions.
But not just those follow these - I...

pseudo code

read sharepoint list - load onto datatable Invoices

using dbconnection to sqldb -- using transactionscope
{
create temp table (#invoices) -- execute nonquery
...

offcourse not - instead of using my code you reverted back to your original one

$File=gci \\abcd\efgh\ijkl | select -last 1
$Attachment = $File.FullName

you may need to enclose the filename with double quotes on the send mail step

$File="gci \\abcd\efgh\ijkl | select -last 1"
$Attachment = $File

I assume that what you intended here was to get the last file on the folder in question - but...

Assuming your dates always have the _ before them the following will work

declare @t table (a varchar(50))

insert into @t values ('c:\abc_20181008.txt')
insert into...

Seen many developers fall under that trap. Wish that would raise an error.

The following is one way of doing it.

Requires that you load the files onto a staging table with a identity column and 2 other columns, reckey and data...

Do they do the queries through SSMS? if so most likely it is the session settings they use.
check the different users on tools-->options-->Query Execution-->ANSI

If through another tool...

you can get the developer edition here - for non production use.
https://www.microsoft.com/en-ie/sql-server/sql-server-downloads

once you tested whatever you need you will have to decide if you need enterprise...

Maybe failing on the snapshot bit which uses different ports

https://docs.microsoft.com/en-us/sql/sql-server/install/configure-the-windows-firewall-to-allow-sql-server-access?view=sql-server-2017

Replication connections to SQL Server use the typical regular Database Engine ports (TCP port 1433 for the...

getting ride of them quite fast - just open one of them, report it 3 times (the same one) and puff all from that user are gone in the wind

Can you supply the DDL for the original tables, not the ones you have altered to add the newid column

If your worry is just the index fragmentation you probably can avoid it if you change your application to generate a sequential guid instead of a "normal" guid.