I was shot back in college, 2008, and took heavy damage to my brachial plexus, as well as severing my coratid and subclavian artery. This left my whole left arm without movement until surgery 2 years later. I had several procedures at the Mayo Clinic in Minnesota where they reconfigured the musculature of the left arm. My biceps control finger flexion, triceps control thumb opposition. A miracle I survived, I know, but I pretty much sat sedentary for the next several years. I got up to my heaviest point on around Jan 1st 2014 and have since lost about 50 lbs and signed up for a century (the seagull century). I started riding my bike (cheap road bike from amazon) everyday to and from work and riding every evening as well. I am currently planning on buying a new bike (2015 Specialized S-Works Tarmac Disc) as a reward for the weight loss but have several concerns.
First since I can't use the fingers on my left hand I need to do all braking with my right. I have never tested the Shimano R785 disc brakes but I am hoping this is sufficient.
Second Since I can not use the left side shifter I was hoping I could use something like Shimano SW-R671. My question about these is with a di2 system can I only use one set of the shifter buttons and place them at the end of my drop bars in place of the cap that holds in the bar tape? If this set up works I am hoping I could remove the entire left handlebar brake and shifter housing, as well as the front brake (since I cant use them) to save on weight.
Third on the left side of my drops since I can't control my hand like I'd like the curvature of the bar can get uncomfortable on long rides. Is there anyone who make ergonomic grip inserts for drop bars?
I don't think Hydraulic is the best option for controlling 2 brakes. On a hydraulic system your brake lever has to travel twice as far and you might run out of space. On a cable actuated system you need to pull the lever twice as hard but it doesn't have to travel further. I'm not an expert on Di2 systems but I think you can use 1 road lever and one TT lever. Cane Creek makes dummy levers for tandem stokers.
It's probably just a matter of time until someone figures out how to go through the full range of Di2 with just 1 lever.
Unless you ride the mountains, you might not need the little ring. You can put a 32 cassette on the Ultegra drive train - and DA too - although the DA might require the slightly longer Ultegra cage for compatibility with the 32 cassette.
Otherwise, I suspect the front derailleur operation can be accomplished with the R671. But keep the RD operation on the standard brifter. Its easier to access and more efficient to use, and probably safer too.
I think a front disc alone is plenty for all but mountain riding. But I would feel more comfortable with two brakes. Just don't know the best way to do this in your situation.