Your issues and experience and everything you detail here is identical to our own from beginning to end. (excluding the custom Layout)
Going back seems unthinkable, and is that even technically possible or cost effective. Maybe for some but is it worth the effort. Maybe going sideways into a similar platform is the best option but who wants to do that all over again, Discourse is a nice platform.
There is an important point here that maybe is being missed. I don’t fully know but it would seems that:
Site Admins would surely pay and easily justify the cost of a reasonable purchase fee for a fully functioning Adsense plugin if even it was a nominal fee. $10/20 with a lower renewal for any full point upgrades with maybe a 3/6 month trial for smaller sites with low traffic.
To support updates, fixes and new features too.
UPDATE Apr 3rd '19:
I wanted to add in this image of Pagew View, Impression & Click for the purposes of illustrating the trend of approx 12/13 days period.
Yellow - Impressions
Red - Page Views
Green - Clicks
It’s very obvious ot see where Discourse went live. 
There is the previous and very stable pattern before upgrade and the jump-off-cliff dive. 
Page RPM, Active Viewable Area & Impression Rpm have performed better, a little erratic but also with some good peaks. Nothing like this trending drop of trailing the sea floor!
Generally site Traffic is performing fine and index errors are continuing to drop like flies with a real rise in valid indexing/coverage report and a vast improvement in mobile enhancement performance.
Naturally it takes time for users to grapple with the new UI and other initial issues but that point is passing now and traffic is really starting to conform to typical patterns, maybe still about 10% lower but the gap is closing, with some tentative signs of a rise in mobile traffic.
Impressions & Pages View if these are down then Clicks are going to collapse. That is the relationship. I you can’t see an ad, you don’t have a chance to click or record the impression.
Blank Ads, are still common and again going back through old threads. There can be no other reason and there is no signs of that improving based on the stats to date despite the recovery and beginning stabilisation of traffic.
Yea give it a bit more time but really I’ve played with Adsense for a long time and you can get results fast if you play and you see the changes within 3/4 days.
Maybe this is an indexing issue and is going to take much longer but I dunno yet. Seeing Blank Ads, Ads not loading but loading on a reload does not instil confidence.