It's hard to help with what you want, because you're basically making an anti-ergonomic KB.
To begin with, the space between key centers should be 19mm. This is according to this study:
Handbook of Human-Computer Interaction
Martin Helander (editor)
Chapter 54, Keys and Keyboards
Part 4.3, Physical Features - Key Size and Shape
Conclusion:
If I take the original KB to compare, we have a 180mm wide overall surface to 11 keys.
Let's suppose this give us 16mm square keys. It's a little worse than standard, ofc, but not that much.
Now let's make your design with 12.5 keys wide. That's around 14mm square keys. Which is basically a 0.75u x 0.75u.
From my point of view, this GPD pocket is already using the limit of the acceptable key size. (or maybe already beyond that)
For what you want, the best would be simply using a bigger tablet (and therefore, a bigger KB).
With a 8" tablet, there would probably be enough space for 12.5 keys wide with 16mm. This is what you want.
With a 9" you could even have 18mm keys, so close to standard that it would be hard to notice the difference.
But this last one is probably already too diferent from the reference.
It's kind of funny 'cause you begin saying "minimum of compromises". I mean, I get what you intended by it (not removing keys) but there will be compromises one way or another. You just choose to let it be the key size instead. We can't have it all.
Don't get offended, but your pratical example honestly doesn't mean much. We can type with the horrible and tiny smartphone KBs (and btw, some of them use the Matrix grid style), but that doesn't mean they are good.
Looks like nobody is interested in such small keyboard. Or I made it PERFECT!
One of the reasons is that you've established so many requirements that there's no much space for changing. So yeah... "perfect".
The other reason I've imagined is this:
polished variant will be sent to GPD (and they probably won't answer, but I don't care).
I mean, honestly, what EXACTLY are you intending to do? Because this could (sadly) end being just another random idea that results with nothing. I'm not even saying this is a bad idea, but I've seen many good ideas that don't "leave the paper". There are any realistc plans of making this happens? If you make the KB yourself to work with any 7"(or 8"
) tablet it would be awesome, but (like you've said) GPD probably won't do anything.