I am delighted you have a new grandson to love and be loved by, it is such a boon to have this happen and I hope this happy event can drive out some of your thoughts on your son. Of course they won't all go, but maybe it will help.
When I say 'your son' - please excuse me for being blunt but you know this already - it is not your son as you remember him, true he is a part of this person, but circumstances, associates and drugs are ruling and he is not the same person at all, and even if he changed would still not be as you remember.
I would imagine any help you give will be swallowed up in the latest crisis and then be gone. I can't advise if you should help, however I'm sure you have realistic idea of what happens - I'm sorry.
If I might suggest you daughter and you block this girl - and any other friends of his that may post on the media. Anything they say is only going to underline behaviour that causes you grief.
Such grief as all those thoughts you mentioned "I lay at night wondering...why ? how ? what can I do ? what haven't I done ? I still border on tears many days" need to be minimized as much as possible.
You do not deserve them and this comes back to your son's behavior, which as you say ripples throughout many.
You said you cannot let yourself think to much of these, may I ask what you can do to head them off?
Looking back I think there is a big difference between putting on a brave face -which you do- and masking yourself. Hard to explain except the latter looks inwards and retreats, the former involves genuine bravery. I hope that makes sense.
I would think those the know you and your circumstances and heart-break would admire you for your cheerful aspect.
You cannot change your son, being torn deciding about future help, or circumstances. All anyone can do is look after oneself.
So I've another question, what do you do to look after yourself? You are a most worthy person and deserve huge efforts to give you a semblance of peace.