You are quite right, disclaimers don’t hold up in court (I’m a qualified commercial insurance broker). However, if you are an employee, the insurance issue isn’t really your concern.
The sort of insurance an individual purchases is not the same as the insurance a company might hold to cover a business. So my advice is based on you being an employee, rather than a freelance therapist holding your own insurance. To cover yourself you should ask for written instructions from your boss. If your boss won’t put in writing (signed and dated) that they want you to treat a client with contra-indicated health conditions, then I wouldn’t treat. Your boss may simply expect you to customise the treatment so that it is safe. There’s no reason why you can’t just do something nice like smooth a cream into the skin. So clarify what they expect you to do or ask for upgrade training.
The finer points of legal liability are not really part of this thread. You’ve asked for general advice about managing a difficult situation at work. There is a big difference between not bothering to do a patch test and signing a disclaimer instead (never justifiable, always wrong) and asking a client to consent to receive a treatment when they have an underlying health condition.
Ultimately as an employee you have to do as you are told. That means carrying out a treatment as well as you can if your boss says treat this lady. Refusing to treat someone in the situations you have outlined could get you fired. You may think you’re in the right but you’re not.