Fix up some codeql/inspection code issues

[jas88]

Fix up some .Dispose/using issues, make finding most recent load ID more efficient

Proposed Change

• Fix up some missed using opportunities and duplicate .Dispose() calls, also inefficient LINQ invocations

Type of change

What types of changes does your code introduce? Tick all that apply.

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update
• Other (if none of the other choices apply)

Checklist

By opening this PR, I confirm that I have:

• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Created or updated any tests if relevant
• Have validated this change against the Test Plan
• Requested a review by one of the repository maintainers
• Have written new documentation or updated existing documentation to detail any new or updated functionality and how to use it
• Have added an entry into the changelog