I'm not quite in a position to advise anyone on physical activity. However, I asked my mom for advice on low impact/seated exercise. She's amazingly fit and trim, unlike me.
I know that swimming is usually recommended as a low impact activity for people with limited mobility or weak/injured joints. Personally, I would love to swim, but at this point I'm too self-concious to be seen in a swimming suit. This may not be a problem for you though. If it is, there may be special times at your local community pool set aside for people with injuries, like how they do for senior citizens etc.. The pool will be more empty then.
Also, there are exercise machines that do not require standing or movement of the legs. As one example, how about those seated rowing machines at some gyms? They are excellent for cardio and muscle-building. Of course, sit-ups are another seated alternative. Also, weight-training machines like chest presses can help you burn calories and build muscle (which will also speed weight loss).
I'm really sorry for your wife's and your loss. Also, for needing major knee surgery. I think you and your wife need someone you can talk to for emotionally recovery and so that you don't fall back on food for comfort too much.
Please don't be upset with yourself. Many people gain large amounts of weight after personal tragedy and mobility-limiting injuries/surgeries. You lost 100 pounds once. You should take it as proof that you absolutely can do it again. Take it slow at first until you get in the hang of things again. In terms of diet, you should probably follow what worked for you the first time as long as it was healthy and sustainable.
I'm rooting for you.