You seem to have two issues - one with support for your child, and one with the head, which may be a result from frustration with the first issue .
It’s not long since I was spotted in a supermarket shopping during school hours. It probably would have looked like my own shopping to any onlooker - I was actually buying something to feed 5 siblings whose mother hadn’t sent in their lunch boxes or let us know in time to order them school meals. (I was Senco so didn’t have a class - but also SLT and as such managed the school when the head and Deputy were both absent).
Needless to say I paid for the food myself - as indeed for the weekly food when we set up our first breakfast club (and the toaster, and the kettle) just as one of my colleagues took a child shopping - and paid for it herself - in school hours to buy him a warm coat because his mother was ill and couldn’t (she was very grateful). We’ve done lots of little shopping trips for the welfare of various children over the years because the children come first. It’s part of what we do to support the children - no reward or praise expected.
At the same time the staff car park is full from 8am (if not earlier) every day, we rarely leave until 6pm if then; we have a meeting after school every night bar Friday; plus parents evenings (2 per term), fundraising events like the school fete, (on a Saturday), Christmas Fairs etc - oh and the 5 days residential each summer where we are never off duty - it’s a full 24 /7 (and the last one with very little sleep as we had a d+v bug going round and as soon as we drove one back home, it hit another - we spent the nights changing bedding, swilling sheets etc out , and washing them.) Then of course there is planning, assessment, etc etc.
The responsibility of the head is even more onerous. There was a day our head was close to collapse on hearing they were now responsible for checking the hot water in each tap - daily - to ensure it was free of Legionnaires disease. Who’d want to be a head with responsibilities like that?
Let’s not also forget the emails from parents (even at 2am) and the calls to your mobile. It’s full on (and I was previously a senior manager in the NHS on a nursing grade - that was far far more relaxed!).
The most important question therefore is was the school covered? Was there a responsible staff member left ‘in charge’? Was it clear that that person was in charge? If so, then what is the problem?
Your issue with your child is different. Have you tried contacting your local authority SEN advisory team - most are still in place if only just - and asking for their advice on the best way forward?