I may have figured out my issue after all this time. Step 5 in the Construct 3 AdMob Documentation:
"Submit your business and payment details to your AdMob account and wait for them to verify (which may take 24 hours). This must be done before ads can be served."
I'm assuming this needs to be done, before even displaying test ads. I just noticed that in my AdMob account, it had a message saying that ads would not be served until the account was verified.
If so, that was probably the issue the entire time... I just submitted my info, and waiting for verification. If that was the issue, I feel really dumb right now.
EDIT: I was finally able to get a test version of the game uploaded to the Play Store. Not sure what changed, but I did the same exact thing I did over a dozen times over the past week, and for some reason it went through this time. Now to wait for AdMob verification, and see if the ads work after that. Thanks again to everyone who has provided helpful input in this thread. I really appreciate it.
EDIT 2: I'm calling this one Solved. I feel like a dumb***, after re-reading the documentation. It's just been one of those weeks *facepalm*. I think it's time to catch up on sleep over the next few days.... Thanks again for your time lionz, and fredriksthlm. And, Happy Holidays Construct 3 community :) I appreciate you all.
EDIT 3: Everything is working fine using the test ad units from AdMob. For some reason I thought ticking the Test box in MobileAdvert object, would just automatically play test ads. Just a matter of the AdMob Account becoming verified, and then I can Publish the copy that was uploaded to Google Play earlier.
Happy Holidays! I hope everyone is doing well. Apologies for any wasted time.