How extensively? That's more a question for the product team than for me. That said, I believe they check the version notes to see if it's addressed issues from there as well as generally checking stability. So more or less the checklist you have above, yeah.

The issue that you may have with the Eador patches isn't what GOG.com can help with, though. We can't delay a game's release (well, that's not completely true. I can think of one example where test team feedback early in a game's beta did end up causing a bit of a delay on finishing the game, but we can't *force* the game to be delayed). When Eador launched, we could either 1) not give anyone access to the game they'd preordered or 2) release a game that needed a little more polish.

1) is really not a good option at all. You paid for the game. You want it when everyone else gets it. The same is true of patches, provided that they pass the checks that our development team has in place. So the testing that we do isn't there to regress bugs from the developer. That's not our job, and it's not what you'd want us to do anyway. Testing a game like Eador would probably take a week or so for bug regression on some of the issues they're fixing. :P.