The design does grant access to the HDMI, speaker and power jacks on the side but currently doesn't allow easy access to the SD card slot. Working on a modification to allow that now.
Yes we had planned on using a header cable and do for the TNCs. We've found (so far) that the Pi's LCD cable is more susceptible to RFi than the GPIO cable but that is from experience with TNC hats and not the DRAWS. We've been able to get past the RFi on the LCD cable by putting a small ferrite core on it.
Our intent is to use the regulator on the DRAWS board rather than the buck regulator separately but as we've got no DRAWS board and we wanted to make the design somewhat independent of the HAT you choose we've designed in the ability for either approach.
The 7 inch LCD is indeed small for a lot of different use cases. In fact I had intended to use it at WFD but found it too much of a strain on my eyes. Something that we will have to think about going forward.