I strongly advise you to report the abusive text messages to the police even if you take it no further at this stage. That way. if things continue or get worse, the police have a record of your complaint and this can be used against them. As for sending people round, what people? Who are they and do they have the right to access your property without your permission? Surely the only people who can do such a thing are the police, and even they have to have a search warrent.
If things continue or worsen, you can get an injunction out against them preventing them having any contact with you or your baby.
However, being able to be apart of their grandchilds life could make a huge difference to their behaviour towards you and their grief for their son.
If you go to your local Citizens Advice Bureau, they can let you know where you stand legally and get you any other help you may need now or in the future. I daresay you are still grieving the loss of your boyfriend too and could benefit from some grievence counselling.
Good luck with everything and I hope it all works out well in the end.