You are right. This is one tough problem.
Here are the steps that I tried take to reduce my Windows EBS volume.
I hope they will save time and effort in further experimenting:
-01- Direct Copy
You create an identical large volume, you create an empty volume of a desired (smaller) size, you attach both volumes to PROD and copy everything from a large volume to a small.
Result: new instance will not pass second status check.
-02- In-place Restore
You backup your PROD into a separate drive and try to restore this content directly into a running Windows instance
Result: it does restore, but all the locked files (registry) and skipped.
-03- Volume Restore
You backup your PROD into a separate drive and restore content into a smaller volume
Result: new instance will not pass second status check.
-04- Files and Folders Restore
The same as previous, but you restore using Files and Folders option
Result: new instance will not pass second status check.
The only option left on the table is to launch a new instance and reinstall and reconfigure all the components one by one. What a pain!