Oh lovey you sound like you are at the end of your tether.
Change your phone number. Only give the new number out to people you can really trust. Buy a very cheap PAYG phone and give the number for that out to your relatives - do not enable voicemail on it and block your Mum's number. That way she cannot leave voicemails when your relatives pass on your 'new' number (which you know they will). Keep the phone in a drawer on silent. Once a day go and look at it and then decide if you want to reply to any of the texts or return any calls.
When the flying monkeys start trying to pull your strings, then say the following:
'This is between me and Mum only, please do not get involved because I won't be discussing it with you'. Be VERY firm and keep repeating and changing the subject. If those relatives refuse to step back then you will need to have a good think about how much you engage with them in the future.
Can you arrange some counselling? It would be very helpful for you to add some perspective and hopefully help with your MH overall. In terms of childcare have a look and see if there is a babysitting club near you. You might only need a couple of hours once or twice a month. I wouldn't be surprised if you saw an improvement in your wellbeing and relationships if you go NC with her.
Gather together all of the evidence - texts, emails, letters - and keep them in a safe place. If she does decide to go down the court route then you will need these to support your case that she is not a beneficial presence in your son's life.
Lastly - and this probably sounds pretty major - can you move? You could do with being somewhere where she doesn't know your address, so can't turn up or send anyone else round there to pester you.