I'm not sure I'm in the best position to advise you as also have a 20mo very dependent on me for sleep! But your post reads very much that you're beating yourself up about it and I just wanted to say don't!
I don't think it's so very uncommon for a 20mo to still be needing a parent at night, especially if that's what they've always known. You're worried it's "not healthy", but it's just how he learnt to sleep, so it's normal for him. My DS is the same - we coslept, fed to sleep etc, so obviously it's no great surprise that he's not sleeping well on his own in his own room. But I wouldn't have done it any other way - it was a godsend for surviving that phase and I loved the cosleeping anyway
Also think it's very normal for a 6wo to sleep better than a 20mo. DS was a wonderful sleeper until 6mo and now is terrible!
Re wanting your bed and husband back. TBH, I would say with a 6wo that's an ambitious aim even without adding a toddler into the mix. Don't mean that to sound harsh, but I've always viewed this time of the children being really small as a time when nothing is really "your own" and if you just accept that, it's easier to cope with than wishing things would "go back to how they were" because they never will
I reluctantly (was due with DS and desperate!) did CIO with my 22mo DD who was similar, and within a few days she was sleeping through. She's been a pretty reliable sleeper ever since ( now 3.5)
Last month (DS 19mo) I attempted a "gentler" CC (going in after 1 min, then 2, then 3 etc) - it worked for a couple of nights then all went tits up and I gave up. Not sure where we go from here.
I think in your position I would find it very hard to do sleep training at the moment because he is, as you say, teething, and that could go on for a while and you're never sure that they're not. And he's just had a major change in his life (a baby brother) and may well be feeling unsettled because of that. Right now, it sounds like he needs you more, not less. But that is just me and I am totally hopeless with having them cry at all and hated every second of sleep training attempts. TBH though, I don't really buy into the whole "scarring them" thing if it's just a few nights to sort it out. Obviously would be different if someone was leaving a screaming child night after night with no change. I think it's more that it's really hard to cope with as parents - maybe parents are more scarred by it!
So if you feel strong enough then give it a go, but in your position personally I would be inclined to wait a good few months now until life is a little more settled with two. You've got a tiny baby to look after too, so don't make life any more (emotionally) hard, if that's what sleep training would do for you. In the meantime you just muddle through with the two of them and work out a way to make it work somehow.
I hope this doesn't sound uncaring, because I really do sympathise and have dealt with two tricky sleepers with v little support from DH myself so I do understand how hard it is. Also, I am frequently despairing of my 20mo sleep myself, so as I say, maybe my advice is no the best!!