With ten finger typing, having the most-used keys on the home row is a significant advantage for speed and ergonomics. With swiping, having a sequence of characters close to each other makes it hard for the algorithm to predict the intended word. With tapping, it’s a disadvantage to have adjacent characters in a sequence on a small touchscreen because it increases the chance of fat-fingering them.

Maybe. I use Dvorak for real keyboards and QWERTY on my phone. I tried Dvorak on my phone and didn’t see any benefit.

Heliboard offers the option. The ideal layout for a small onscreen keyboard may be rather different from one for typing with all your fingers though.