Hi and welcome to TTL
First of all im sorry for your loss.
As for his family and this is only my opinion but they are totally wrong. It is not your fault your ex felt he had to do this. Please stop blaming yourself no matter what others say. Even if you had answered all of his texts or returned who is to say he still wouldnt of commited suicide?
Its totally out of order to blame his death on you and i hope they soon realise this.
I dont use Facebook but i would of thought threatening behaviour can be reported to the site or if it warrents it to the police?
As for the funeral i know you probably really want to go to say good bye and pay your respects but if there is any chance of there being trouble i would advise you to stay away and visit in your own time when others have gone.
Reading your post i can see you tried really hard to get him to ask for help. You did everything you could at the time. There was no way you could of known what he was planning.
I hope you can stay strong for yourself and children during this sad time.
Please keep posting if it helps