I guess it depends what the aim of the exercise is - if it is to get your heart rate up and give your muscles a workout, then swimming is fine. And you sound as though you are doing loads against all the odds.
If it is to try and increase your bone mass, then weight-bearing exercise is the thing. But it has to be tailored to your capabilities, and this is where your GP is not thinking. To suggest you go up and down the stairs so many times is clearly silly, and he/she will finish up having to give you more pain relief if you do that - or sending an ambulance to mop you up from the bottom of the stairs!
In your circumstances then simply walking as much s possible and sitting as little as possible is probably the best thing to do.
Or you could ask your GP to send you to a physio for advice on weight-bearing exercises that you can safely do.
Reducing contact because my heart just isn't in it.


