Hmm I guess it's hard to give advice without knowing everything about your injuries.
With light weights being a little too heavy, is that because it hurts?

For me with my injuries it always hurts, even after 6 years of training. I just got to will myself through the pain and keep pushing myself.
Although if your not going to lift really heavy weights like me the pain should become bearable fairly quick.

If you want to build strength in your arm I would start light and do full body workout, most of your upper body exercises will also help to strengthen your arm. So back exercises etc/
Focus on light weight and range of movement. Then gradually increase weight slowly.
Also give it time, as it will take a while to build up that strength.

I would consult your doctors and keep him informed of what your doing, as long as they give you the go ahead to get in the gym you should be fine to keep pushing yourself.

It's not easy to push through the pain all the time, most people with serious injuries like ours will not stick it out.
If you can stick it out though your pain threshold will increase and you will be able to tolerate it better each time, so for me I can tolerate a fair bit of pain and that's how I can get through my intense workouts.

If you can't do weights look into hydrotherapy, that's where I started :)