Hawthers how terrifying!! You poor thing! Well done for such a perfect response under pressure though! You really are a supermum!!! I hope your DS1 is fully recovered from his fright, and you as well!
Dream we had a similar response to the dummy here when we were trying to get DD to use it. She'd fight and complain loudly for a while and then suddenly drop off. With DD though she was so young that it would have been weeks before she had the coordination to keep the dummy in herself, since she doesn't seem to have an instinctive knack for it. DT1 is several weeks older and hopefully if you persevere with the dummy he might learn how to keep it in himself (once he realises how much he likes it). Whatever happens though you are not doing anything wrong!
I remember the first few weeks, when S was sleeping moderately well (for a newborn!) but B was up all night with colic. Then B seemed to improve but S started playing up at night. You've done everything right with both of them, but they've just had separate sleep problems at separate times. If you had only one baby you would have had a few weeks of slightly better nights, either at the start or more recently, it's the tag-team effect which is wearing you down! Don't let it affect your confidence though, you're such a wonderful Mum to all three of your DC! I really hope you start getting more sleep on a regular basis soon!
By the way, if it makes you feel any better I do EVERYTHING wrong (rock/feed/walk to sleep, zero routine or consistency, over-stimulate, inadvertently encourage snacking, etc etc) and DD has still had a few good nights recently. Not last night unfortunately. I think they'll all settle better as they get older, even if we're not textbook Mums!
Scarlet sending lots of hugs your way as well - what a stressful day! Poor you and poor DS. How did the day go in the end? Did he feel better after his nap? Was your friend able to take your mind off all the worry? I'm No Brooking hard for your counselling appointment to really help to take the scary edge off of things for you. Personally my bugbear is the whole SIDS thing, last night I had a horrible dream and randomly woke up an hour after FINALLY getting DD to sleep (at 2am, for the first time that night!) and I almost woke her just to check she was okay! I settled for resting my hand on her chest and counting her breaths instead 
Too tired to go through and name check everyone. Sorry, I had sooooooo little sleep last night I need to get some shuteye before my shift starts at 2...