Late to thread - glad your DS is doing OK. I had something similar 7 years ago - no vomiting etc but chest pains and rang 101, sent paramedics and they took me to A&E, raised troponin and said I had had a heart 'incident'. Ended up staying in for a week and having ultrasound, Xray and angiogram, then they let me come home saying it must be a viral myocarditis and they thought I was fine, but would do an MRI to check.
The MRI was months later and after that was clear they took me off all the meds and said it didn't count as a 'heart issue' and didn't need declaring on things like holiday insurance going forwards.
But the cardiologist did say when they discharged me that I might well carry on having twinges for some time, and not to worry. And to be fair I did - maybe for a year or two, on a very minor level.
I realise my experience wasn't quite the same as your son's - and there was no mention of an issue with a ventricle - but they were pretty chilled like your consultants seemed to be, and said similarly - any issues speak to your GP. I just thought this might help to reassure you somewhat?
In terms of how they treated your DS and their lack of understanding of his ASD - I would definitely take that further. We all know how busy hospitals are - but some basic training could have made his experience so much better - and the lack of it could have triggered something far worse like a meltdown or him discharging himself when he shouldn't! Which as well as being awful for you and him, could have had long term ramifications, and could also have led to more time-consuming work for the staff involved so lose-lose all round.....