I'm not sure it's the right place, but filing an issue at crbug.com might at least be a good place to start. It's the bug tracker for Chrome, but it also covers the WebView. If you explain the problem with the WebView app not being able to update, they might be able to do something about it.
I think the weirdest thing is if it worked with v71, then from what you've said it sounds like failed updates beyond that have actually reverted it to the factory v43. A failed update should have at least left it on v71!