Hi Tom,
See my post here for some feedback on differences between iOS Scriv and the desktop versions. Particularly, that post links to another, much longer thread you might find useful to understand the strengths and shortcomings of iOS Scriv. Also see this thread.
The short version for me is: I find iOS Scriv really good for writing drafts, but much less useful for research, planning, or extensive editing. (No Quick Ref panels, Collections, Keywords, Custom Metadata, or Automatic Backups. iOS Compile is completely different/disconnected from desktop version and extremely limited, but that would only be an issue for you if you needed to compile while mobile.) Other posters may feel differently. YMMV.
It seems you are mostly concerned with touch capabilities of iOS Scriv, so perhaps you could elaborate on your challenges using the Surface Pro as a laptop when you go mobile? I’m asking this because your post makes it sound like you’d still be carrying the Surface Pro along with you anyway. If I had both my iPad and my laptop in front of me, to use Scrivener I would always reach for the laptop first. And if I was planning to do extensive writing on my iPad, I would always use an external bluetooth keyboard, which would be yet another device to carry around.
To answer your specific question: iOS Scriv’s touch interface works very well for me. (I only use fingers, no stylus.) It would have to work very well, or else no one would buy it. 
Best,
Jim