Hi,
Welcome to our new Microsoft Q&A Platform.
>>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:
https://www.catalog.update.microsoft.com/Search.aspx?q=windows%201809
>>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:
https://social.technet.microsoft.com/Forums/en-US/home?forum=winserverwsus