I hear you there...
I may not know how you feel exactly but I feel you...
I saw that kind of attitude many times and I really don't like it, you're not alone with this kind of situation.
The person I know with this kind of situation too was worse than your daughters bf coz her hubby was nothing but a "professional bum".
Your story hits home coz I care so much about my friend but she seems to get used to that situation. C'mon this guy should be man enough to work his ass off and start to dream bigger not just for himself but especially for the children. If I were in your situation I might have done the same thing and perhaps even worse, I don't wanna see my loved ones suffer without me giving a damn. No no not in this life time.
I see that you've been keeping everything inside and tried to help your daughter by helping her bf get a better job. I can see also that you have a sensible daughter the way she reacts towards you when you shit her bf, I think this domestic issue is a major thing that your daughter and her bf should talk about. Talk to your daughter as well and be the loving mother she needs, present the pros and cons of their current situation, tell her that you care so much about the future of your granddaughter and only want nothing but the best intentions. you've had your fair share in life and I think that's enough for your daughter to mirror with. You are a though woman as I see it and I admire you for that.