My situation is similar but my starting point may be different as I chose to have DCs alone. No help ever from the v start and one DC has Asperger's. They're twins and 12 now. Absolutely no respite in all that time and although it gets easier in some ways, it gets harder in others.
I too am freaked at the thought of something happening to me and the DCs having no one. Just next week, I'm compelled to work away for long days, at a moment's notice and unsure what to do with last minute arrangements for DCs. Although 12 might appear to be quite old now, because of the one with SN and the fact that they've never done much at all for themselves, there's not easy solution if I'm not there to do school runs either side of the day.
I will call in help from a friend, in an emergency - but few people want to have my DC with Asps. as he doesn't fit well with other children. Also, I hate being beholden to others when I can't easily reciprocate.
I feel I just have to stay strong ALL the time, until they're old enough to be independent. Whenever I've been really ill - thankfully not needing hospitalisation - I've 'managed' them from my bed but still had to drag self out to do school runs and buy them food, even when barely able to crawl around.
It's tough. I adore them and wouldn't be without them but I actually find it harder now than when they were little, as their needs are more complex and after 12 years, with no breaks, I feel exhausted.
It's also difficult to build up a social network as all I ever do is work or look after DCs whilst so many other parents get occasional hours off or even whole days off, whilst a relative or OH takes over.
I don't wish for a partner but I do wish for a few other adults in our lives to shoulder the responsibility sometimes or just take one of them out - or better still - both of them.
So I do understand how you're feeling and I've also had horrendous neighbour problem in the past too! Sometimes I wish I could 'hire' a man to scare off neighbours/ exploitative workmen/ people taking advantage of a woman alone eg at the garage!