Mine used to have a kalloy adjustable angle quill stem (a la Chegs) but it was heavy and I got it really so I could find out what angle was good with the hope of replacing it with something lighter and fixed. I opted for this KF stem
http://www.fisheroutdoor.co.uk/publi...mlnndd7h7bpdv6
which works well with my aero bars. I'm not that tall though (5'5") so this may not have enough rise or reach for a taller rider. Of your suggested options I think going for C will give you the most flexibility as there are a million and one different A-head stems out there; it will be chunky looking though and may look top heavy on a twenty. In terms of elegance the recumbent stem is the winner though reach issues may again be an issue.
It's worth noting that the longer the reach beyond the pivot point the more responsive the steering will feel. Coupled with the small wheels I'd be careful not to have too long a reach as it will feel squirrely and hard to keep straight.
It's great to see your project shaping up - thanks for canvassing our opinions and keeping us up to date with the latest progressions (is looking forward to seeing the shiny finished result!)
Best
Huw