Carpe, your posts have made me laugh, not at you but with you. You have a way with words.
That place you went to for lunch sounds good, could you organise a lunch with a few of the other mums and their kids to go there one day?
Or do something at home, invite someone over for tea and be open with them, the way you have been on this thread. I've noticed that much like on MN, in RL, people are always willing to hear you out and be sympathetic if you have an issue. My son had major separation issues, like you I couldn't go to the loo without him stressing big time even if I was at home or he was at a play centre he'd been going to every week for weeks on end.
But I found that if I spoke to the other mums about it and say I was struggling with it a bit, they were really sympathetic and supportive.
People like to impart their pearls of wisdom and feel wanted or needed.
Don't mean turn into the local whinging whiner but maybe open up a bit more, it sounds like you might be more reserved than you mean to be. Sometimes that can come across as aloof, unfriendly or possibly even arrogant which you sound far from.
If people walk off mid sentence, they are not the people to be befriending.
Talk to the person you think is most likely to be sympathetic. You could start off by saying, don't know if you've noticed, but my ds is quite clingy and insecure if I leave the room. I'm trying to help him overcome it, your child seems confident, what do you do at home if you have to go to the loo or something, I'm really struggling with it etc etc.
It's not necessarily that you're looking for advice but it's a way to open things up and get more in depth conversations going.
Or start off by saying, I need to pick someone's brains, and then ask whatever it is you want to ask -people are very receptive usually if you have something specific to say/ask.
If small chat doesn't come easily to you, wouldn't suggest you try faking it but try and move the subject to a topic you can talk about comfortably.
How about forming a book club if you can't go out in the evenings but assuming some of the people near you can get out and leave dh/dp with the children? You could ask for some book recommendations and gauge who likes reading and then sound people out on whether they'd be interested in a book club or a film club, organise a night out when you could go to the cinema and your dh look after your ds.
I'm an organiser and a doer, I know it's not something that comes naturally to everyone but maybe get the ball rolling a bit yourself if you can, no guarantee that people will reciprocate but you never know. You mentioned that people haven't returned playdates, that's a shame and quite rude. Again people to avoid if it's a regular thing but if you genuinely like the person or your son likes their child, wouldn't give up on organising playdates, if you repeat an invite, people usually twig and say oh hang on, it's my turn to host etc.
When you have a clingy child, it can be incredibly draining. Hope you can get some time to yourself to do other things, when your dh can help out with your ds otherwise you really do lose sight of who you are too.
You mentioned you like baking -maybe ask people for some recipes and say you want to try something new out and ask if they want to come over and try it out with you, but choose the people you genuinely like and want to develop a friendship.
You sound like you would make a lovely friend. Be selective and discerning, you don't sound like you're desperate or need to be, it can be lonely working from home and looking after children but doesn't mean you want to be friends with anyone or everyone!
Decide who you like and if people from the playgroup aren't on your wavelength, agree you need to consider other activities until you find people who are on your wavelength and have your sense of humour!
Cripes, another bloody long post -sorry! It's a quiet work day today as the UK's closed!