This turned out to be self-resolved, but would be good to get some feedback.

Summary:
1. Starting today, after all working. The GCE plugin seemed to regress on spawning windows instances. Linux instances were ok.
2. I got things to work by
a. removing the service account associated with the ephemeral node.
b. toggling the use of instance templates and directly using the image.
3. Additionally, it appeared that my Windows? section where the credentials were stored were missing. I added them back in.

Additional Notes:
I recently updated (on friday) the following plugins:

shared groovy libraries
 groovy
  multibranch 
 bulid step

Details
I would get messages such as:

• "message": "The resource 'hostkeys/' of type 'Guest Attribute' was not found.",
• Could not retrieve subnetworks
• Missing or empty node.

I went through related JIRAs and applied them. That didn't seem to take.

Filing this as minor. I'm wondering though if anything in my upgrade steps may have regressed my config.xml setup. probably not, but i'm just wondering. if not, feel free to close.