From the DAFNE course, I was taught that at 2.8 your brain can literally freeze, and anything can happen from that point on - I think the term is "neuroglycopenic effects", ie the brain is starved of fuel and it can all go very pear-shaped after that.
From my own reading - I'm 2yrs into it like you - 2.1 or 2.2 is the level where fits can start, and 0.55 is when coma sets in. Having said that, as others have noted, it's different for everyone, and I remember reading a paper about a man who was perfectly lucid and had no cognitive impairment at all with sugars of 0.5, so he was written up in the medical literature. For brain damage, I think you might need to be in a coma for a really long time, so don't panic too much about that. Having said that, people can easily damage themselves during a hypo even if the hypo itself caused no harm, so you should take care not to let it happen as far as humanly possible. When I was diagnosed an old Type 1 friend came over (he's had it about 30yrs) and told me it's not usually the hypo that hurts you, it's what you do on the way down. So you should aim for prevention rather than cure.
My lowest was 1.9, it was only an hour after injecting, I'd even knocked a unit off my normal dose for that amount of carbs to be on the safe side, as it was near bedtime. An hour later and there I was. Over the next 20mins I drank about 3/4 a litre of Lucozade but only made it up to 2.1. I didn't know it was 20mins until much later though, because my consciousness was so impaired that I could read the time but not subtract the time of one reading from another and work out how long this had been going on for. It was the middle of the night, I was alone, I tried to call my neighbours unsuccessfully and took the decision to call an ambulance because I was losing my ability to keep going and for all I knew I'd been like that for hours. They arrived within about 10mins, and it was only as I saw the paramedic that I finally felt my brain 'click' back into reality, and I tested at 6.1. At the time I was mortified but in hindsight I'm so glad I did it, as I was in danger. It took me half an hr to be able to string a full sentence together even though my sugars were constantly rising and to this day I don't know how I made it down the stairs to open the door for them - I remember blanking out briefly a couple of times on the way down. I apologised to them a lot, and I asked basically the same question as you - did I call you too soon, is it normal to feel that bad at that level - and they said it differs from person to person and they've seen people flat out on the floor with sugars in the 3s. They were extremely kind and assured me that they were here for people like me, and no I wasn't stopping them from dealing with things like heart attacks and all they'd been called out to so far was two drunks so they were more than happy to look after me.
Some hypos will hit you really slowly and are very easily sorted, and some really quickly. You'll probably know when you're dropping really fast - you might get all trembly inside + outside, or very very shaky indeed and be unable to get the lid off a bottle, and feel like Alice in Wonderland after drinking out of the magic bottle, see lights, have your vision go weird, feel sick, start crying, or sweat profusely for example. Sometimes you might not recognise you're going low until you're already quite hypo - I've discovered I was at 2.4 before when I went to wash my hands and they seemed a bit odd-looking(!) so I tested cos I thought that was weird. Only then did I get the full-on symptoms - it's not the first time that has happened to me and I've heard others say similar things have happened to them. I've had all kinds of symptoms, a whole raft of them - they're not only different for everyone but for me they're different every time.
Four is the floor - ie, if you're at 4 you need food, now - and if you find yourself between 3.5 and 4 on testing it can be classed as a 'borderline hypo' - ie, forget cereal bars and go straight for the quick-acting carbs - Lucozade or coke or orange juice (refined and simple carbs work very quickly compared to complex carbs) and when you're back up to 4 or above then eat something else like the cereal bar to keep yourself stable. One hypo can lead directly to another if you don't bump up your levels appropriately with long-acting carbs. Remember that quick-acting carbs are quick-in and quick-out - so if you've had too much insulin their effect won't necessarily outlast the effect of the insulin, depending on how much you had when. So eat when you're above 4 again. When you're below 3.5 you need to sort it out with no messing around. And you can sometimes be fooled into thinking you're dropping slowly, but testing reveals you're not, so please always take it seriously.
As regards sitting down and waiting for someone to get food for you - no, never. That shouldn't happen. You are the diabetic, and it's up to you to be prepared. Don't go anywhere without Lucozade, Glucogel (you get it on prescription), a mini can of coke or two, or some other kind of quick-acting carbs. It is your responsibility to carry this with you at all times and if you fail to do so and something serious happens, it's 100% your fault, like it or not. Put it this way, if your wife had severe asthma and went out of the house routinely without her inhaler, then ended up in hospital after a massive and life-threatening asthma attack, what would you think, how would you feel, how annoyed would you be? To be honest I am upset on your behalf that you have not been told this by your diabetes team as it is basic rule number 1 and you're really lucky not to have been caught out by it - I had this drilled into me the day I was diagnosed and several times since, and it's information which has essentially saved my life. Please put Lucozade in every room in your house, and always but always carry something with you. I've discovered the joys of little packets of fizzy cola bottles for those hypos between about 3 and above that I don't think are going down too quickly - otherwise, I scrap that and go straight for the liquid. The little packets of sweets squash down easily into pockets and bags and take up almost no room.
If you go to wikipedia and look it up there's lots there. My manager is Type 1, he's had it most of his life, and he's never passed out yet; I've only had it 2yrs and had a couple of scares, but I've also never passed out yet. Both of us always have Lucozade everywhere and in our bags/rucksacks at all times. To get to the point where you 'just keel over' you've probably got to have hypo unawareness, which is when your HbA1c is so low that you lose your symptoms or sometimes it happens when you've had diabetes for a couple of decades or so - and from what you've said, it doesn't sound like that's any kind of problem for you as you have symptoms around the 4 mark.