Test your app end to end. I've recently used distributed replay to do similar, though I was looking for performance regressions due to the optimiser changes in SQL 2014.

Being in compat mode 90 does not stop you from using 2012 features. The compat mode is only there to tell the query processor how to handle syntax where the...

Because there's nothing wrong with that syntax.

https://msdn.microsoft.com/en-us/library/ms175874.aspx

Rules for Regular Identifiers

The names of variables, functions, and stored procedures must comply with the following rules for Transact-SQL identifiers.

...

The CASE prevents any index usage on the settlement_key columns. No hints or columnstore is going to change that. If the join is the cause of it being slow (and...

Because SQL's execution model is parse, bind then execute. The entire module (procedure) is parsed and checked for validity before any part of it starts executing. The incorrect column errors...

Always, for everything that's going to production.

Ok, so not a copy-only and it looks like they finally took the notruncate option out of maint plans.

Have you tried increasing the frequency for log backups?

You need further calculations (which a google search should turn up, I don't have code on hand), or instead go and look at the counters in perfmon instead.

To start with, unless you're using several thousands of characters of XML for login names, this piece isn't going to work correctly.

SET @data = EVENTDATA();

SET @login = CAST(@data AS VARCHAR(max));

EXECUTE...

Providing you have both the mdf and the ldf, it should be fine. Attaching just the mdf will fail.

Screen shot of the properties of the log backup task please, the stuff you specified is completely unimportant.

Availability groups shouldn't cause a LOG_BACKUP wait.

No, you don't need to shrink it. Shrink only removes empty space from the log, so all you've done with the shrink is removed the free space SQL would have...

Check the headers of the pages which have the page locks at the end of the second insert. See if they're pages which are no longer allocated. Pages deallocated by...

We still haven't answered the most important question.

Is this query a performance problem?

If so, have you tried the usual methods of tuning queries?

You're suggesting complicated and unusual 'solutions' to something...

I'm not spending another couple hours redoing this all on 2012. Not on a Saturday evening.