Oh Cosmosis - you poor thing. What a hideous night. I've been there, and I think a few others on this thread have too. It's awful.
Firstly, you did absolutely the right thing putting him down and walking away for a bit. I've had to do that quite a few times. I've also been frightened sometimes at how angry and frustrated I was feeling, and have felt guilty and awkward around DS the next day. I think it's perfectly normal and natural in the situation - sleep deprivation can really push you to your limits. But it won't have done him any harm.
Secondly, you are not a crap mum at all. On the contrary - you're a loving mother who can't bear to leave her baby to cry - even for 15 minutes. You've been suffering sleep deprivation for months and you could have just shut the door on him, poured yourself a large glass of wine and let him scream himself to sleep ages ago, but you haven't - you've comforted him in the night, you've done everything you can think of to gently encourage him to sleep better. That doesn't sound like a crap mum to me - though I understand why you think that because I've thought it about myself many, many times.
I wouldn't blame you at all if you did CC. It sounds like you've tried everything, you've put up with a lot and you need your sleep now you're back at work. To be honest, if our current plan isn't working before I have a nervous breakdown by the time I go back to work, we will end up doing it too. It didn't work before, but maybe it will when our DS is older. Some babies just don't respond to the gentle methods.
Have you tried just sitting next to him and comforting him in the cot? (Can't remember how you're settling him - know you're not feeding to sleep anymore.) That's how I got my DS to settle a bit more easily. It did involve crying but not excessive amounts.
When is DH back? Can anyone come and help you today while you rest?
Poor you. Just remember, you did the right thing and you're not a crap mum at all.