Cinny, I am a stay at home mum too. My son is 8 now so life is much easier for me but I'm just popping in to this thread to say that I do understand your difficulties too. You are living life in a very taut state of existence in which you only manage by planning and managing every second.
No, you should not have to deal with Monster during your precious time at the yard, I hope the barn owner is sorting it today....or sooner!
I doubt Cowgirl was trying to be hurtful - until you have lived life in another persons shoes it is sometimes impossible to imagine how hard things can be. It WILL get better! All those classes and groups you are taking your son to, all the coaching and caring you are doing - will reap rewards as he does learn to live more independently over time.