Thanks for using Auctiva, I wanted to provide some information about the questions you posed, your original questions/comments are in italics.
1) Do I now need to have the UPC fields populated in order to post to ebay through Auctiva?
eBay began requiring product identifiers (GTIN, which in your case is UPC) within certain categories last year. In 2016 they noted product identifiers will be required for multi-sku items and in a number of additional categories. So in some cases/categories, yes it's possible that UPC is required, but it's an eBay requirement, not an Auctiva requirement.
I was unable to post last night presumably b/c these fields were blank.
Possibly, the Auctiva Scheduled Listings notifications page (http://www.auctiva.com/account/events.aspx?filter=3) should give you exact eBay messages as to why a listings failed to post. If it failed because of a required product identifier, you should see that in the message, probably under the link "View Error MEssage From eBay"
2) As a test, I populated the UPC's for one multi-variation listing and posted it to Ebay. When I view my listing in ebay, I do not see the UPC codes in the Item Specifics or anywhere else. Am I missing something are will the UPC's not be visible to potential buyers in Ebay for MV listings?
The UPCs are not always visible in your listing on eBay, but should be used by eBay search, i.e. if a user searches for a specific UPC from the eBay search, your listing should be returned in the results.