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If GOG is playing seriously with this attempt to create a multiserver/multistore launcher it needs to be more transparent to the user. The outcome when you choose install any game of your library have to be one click install. Just launching the external launcher and directing to the install window of that launcher is the same thing previously made by apps like playnite. I know there's some problems with the necesary recognition/bran visibility of diferent platforms but it's something that could be adressed by making deals with other companys like EA/UBISOFT... etc to improve interactions/API. It could be adressed improving the API in other launchers just triggering a unified GUI that makes visible THE EXTERNAL COMPANY BRAND and a automatic switch to that or just using for launching as a tiny client version.

It's necesary to stablish some standard in the industry to change this fragmented industry to make posible that the interaction of diferent libraries isn't the actual pain in the ass. Maybe offering GOG store as a global marketplace for any other stores could be a good deal to negotiate launching and management fullly controlled by GOG if the user wants it.

Still waiting. Nothing diferent than Playnite still.

And please. BIG PICTURE MODE! PC are the modern console choice for many people. Is prehistoric nowadays to manage instalations and apps with mouse as the only option.
I installed Dishonored 2 on Steam yesterday, I just saw the Steam install screen and nothing else. So I don't understand your issue.