You have received many great ideas to address your particular issues.
From my perspective, there is a broader array of things to consider.
.
What are the requirements not only do to your physical challenges, but
your riding style and cycling area requirement as well?
Flat, some hills or San Francisco?
Are you looking at going to the grocery and around the block or are you interested
in doing club rides at pace?
What are the likely demands you will be putting on the system.
All weather year around or a few rides during the season as time permits?
Also to consider would be what is your physical shape and how strong are the remaining
digits? How flexible are they?
Electric bicycles are an option. Some you have to pedal and can assist up the hills,
eliminating shifting. Others have a twist or push type throttle or a push button to
engage the motor when needed, limiting or eliminating the need to shift.
To facilitate braking there are some with a regenerative feature. The the motor
converts to a generator, and there is drag introduced in the hub as electricity is
produced, slowing the bike down. This can keep speed constant, even on a long
grade.
There are some really out of the box solutions I have seen. A rear brake lever mounted
on the down tube and foot activated. Another bike had a brake lever mounted on the
non-drive side chainstay, activated by the riders heel.
I have seen brake and shift levers with paddle type attachments. There are sometime
solutions which are simple and use common things to adapt
One young boy, born without digits, had handlebar cuffs made for him out of pvc and
a waterbottle cage design. Another person used an old fashioned car steering wheel
spinner to help make riding better for them as they had only a single arm.
Depending on your location, a problem solving LBS may be your best bet for this. They
likely have seen many iterations of bicycles, "customized" for any challenge a rider may
have. Many generally have the "stuff" needed to take on special challenges, and will be
familiar with your riding area. Especially off season, when they have the time to put
some thought into making your ride safe and enjoyable.
Many flat-free miles.