Great stuff. The TwinView Dock is definitely one of the biggest draws to the phone for me, as well. I use it mainly for running two Pokémon GO accounts at once, with no need for a second phone or split screen. 🙂
I do agree that there is more potential for the accessory with some software tweaks and options.
I totally agree about needing an option for portrait mode. I've requested that in a thread I made previously: https://zentalk.asus.com/en/discussion/44182/twinview-dock-questions-suggestion
I really like the idea of the "single screen mode" you suggested. as it stands, using a keyboard while in landscape mode is really awkward, due to how much space the keyboard takes up. But if it worked like you suggested, that'd no longer be an issue. Using the bottom screen as a virtual controller for a game on the top screen could be pretty useful, too.
... Somewhat related, but I wonder if someone's attempted getting Nintendo DS/3DS emulation working on this using both screens... that could be pretty cool.
... And I don't know how practical this would be, but perhaps a different kind of "single screen mode" where say, a web browser is stretched across both screens, giving you a bigger view of a webpage.
Additionally, Asus should be trying to reach out to more developers to add support for the TwinViewDock. It's kinda sad how few games actually support it (and honestly, I'm not interested in any of them).