What should one do who is unable to sit between the two prostrations for fear of harming their knees? Is it permissible for them to pray sitting on a chair, and is their prayer valid without hearing the call to prayer (adhan)?

You are in grave danger by abandoning prayer, for it is one of the greatest destructive sins, and no one has an excuse for abandoning it. Rather, it is obligatory to pray according to one's ability when the time for prayer begins, whether by hearing the adhan or by knowing the time through other means. As for sitting on the left foot, it is not obligatory, and you can pray in any manner that does not cause you harm. However, it is not permissible to pray on a chair if that prevents the obligatory prostration while you are able to perform it.