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>>Is there an MCT (or another way) to update these workstations to 1809 without having to do a clean install?
There is no MCT that could download 1809 iso for upgrade now. What you need to do is to find out a 1809 iso for in-place upgrade.
You can download the package using the following link:
>>When we do get our WSUS up and going, will it be able to pull down past feature updates, not just the most recent?
In my experience, the answer is: yes. But I still recommend you to ask the WSUS forum as they will be more professional on your issue.
Here is the right link: