What you propose to do, to Accept the DVO on a NO ADMITTANCE basis, sounds very sensible. Legal actions are holes into which you pour money, not necessarily with any outcome at all, let alone one in your favor. I strongly suspect the figures quoted in fees are way under.
I'm sorry about your son he sounds a rather easily led person and really asking you for money by texts having taken out/supported the AVO is just not on.
If he came face to face, and talked it might be more hopeful, however it sounds as if you are simply being used.
Do you feel compelled to just hand over money, or can you use it as a lever to have him visit and work towards getting things changed?
Look, many people in your situation, particularly with a history of PTSD (which I have too) can get overwhelmed, if either of you get so down you feel like taking your lives then you need a plan, something you do not have to do any thinking abut or decision making at the dark time.
First I'd suggest you both have a safety plan each like I use, there is a good one here for a smartphone you fill in advance, preferably together.
You fill it in in beforehand with all the things that you might enjoy or make you feel calmer. It will be different for each of you, when I did it my partner had to help, as I could not think of those things by myself.
I have books, music, visiting a friend, movies and comedy routines from an Irish comedian - humor helps me.
If all else fails and you want the comfort of a sensible human voice I personally suggest the Suicide Call Back Service (1300 659 467). Organizations like that one exist for people like yourselves and giving them a bell is expected. It is free.
Please let me know how you go