My son has (bullous) EHK (palms and soles affected) but from birth for about 2 months his soles were beautiful and soft and I assumed they were unaffected all that time - so I don't know if that helps or not but I would say (at least in our case) that ultrasound wouldn't show...
I would be really interested to hear how your scan goes if You don't mind posting again - I've been thinking of going back to my sonographer to have them re-look at my scans (twins, so I had lots of scans) retrospectively to see if they can see anything - as much for the sake of educating that industry as anything.
As far as birth plans go - I opted for vaginal but ended up with a c-section (breech twins) after 12 hours - Harry's skin was SO fragile and thin that my personal opinion would be try for vaginal BUT if it looks like you'll need vontouse suction or forceps etc then opt for the c-section at that point... just my opinion but maybe someone who vaginally delivered a bullous EHK baby may say otherwise...