Hello, I would be happy to help.
You are clearly suffering from a condition called vaginismus, but you probably already knew that term. Here is an excerpt from a reputable source with some thoughts regarding treatment to start this discussion:
Primary and secondary vaginismus may be treated initially with myofascial release of muscle tension in muscles of the pelvic floor, thighs, and abdomen, with or without biofeedback (you have tried this with botox). Desensitization techniques are then applied to give the woman control over muscle tonicity/relaxation. Activities include Kegel exercises, the pelvic floor drop technique (reverse Kegel), and use of dilators. Self-insertion of dilators of gradually increasing size in and out of the vagina teach the woman that control of introital musculature can be voluntary and painless. Desensitization is the goal of dilator therapy, not physical enlargement of the vaginal opening. A small amount of topical anesthetic should always be used on any object inserted intravaginally since desensitization will not occur without painless insertion.
Other therapeutic approaches include sex therapy, progressive relaxation, sensate focus, electromyography, biofeedback, use of benzodiazepines, hypnotherapy, and Botulinum toxin type A injections.
So, with regard to masturbation, I think that this might not be the right approach to learning desensitization techniques. No, gentle masturbation will not weaken the pelvic floor muscles, and, in fact, this is actually what you are trying to do. You obviously know the triggers involved in making things worse, and the feeling of 'fatigue' is related to the fact that the muscles are in a constant state of contraction.
I have found with patients that the use of Physical Therapy is an excellent place to start. This involves intensive therapy to learn biofeedback techniques used to relax the muscles surrounding the vagina. I do not think that masturbation is bad, but it should not result in feeling worse afterwards.