TBH I disagree with most of the early responses and I don't like sleep training and would prefer (personally) to breastfeed an 8mo to sleep.
Sometimes in parenting you have to go out of your comfort zone, and try things which you don't feel totally happy with. Sometimes I think this is imperative for your relationship as it fosters respect and teamwork. Sometimes your instincts aren't the only way, and sometimes you have to let your partner try things even if they don't work. It would be impossible to parent and never do anything which didn't work. It's not the end of the world if something isn't right. The hugely important thing is to keep connected and communicative and stay respectful to each other. If he is so distressed by the sleep situation (and honestly it sounds like you are not really happy with the set up as it is anyway? You say in the OP it would be "amazing" if it worked) then it is reasonable to try to tackle it. Sometimes you need to compromise.
I don't think that the methods your DH was using would harm her. You did not go straight from co-sleeping to a harsh cry it out regime - he was with her, he was comforting her, he was distracting her, he was not being angry or aggressive or unreasonable, and you described him as "doing brilliantly". Sometimes a different approach is beneficial - children do have two parents and one does not automatically trump the other, unless what one is suggesting is abusive/neglectful.
It is reasonable to tell him how you're feeling, though, and I am a bit concerned that he became so angry about it. That's a different topic but you do need to be a team on this. It sounds like you have collaborated to some extent - you've agreed to try the sleep training method even though you're not happy with it, but (you/he/together) have found a sleep consultant who doesn't use leave-them-crying techniques, and he's the one doing the bulk of the work and it sounds like doing it to the instructions incorporating another method of settling etc, not just being half hearted and leaving her to it. That's all positive and sounds like a collaborative approach and I think you did EXACTLY the right thing in supporting him with it by staying away even though you had reservations. He needed space to try out his method and you gave it to him.
The anger on you expressing your feelings about it/general difficulty discussing emotionally charged topics with him does sound as though it needs to change, though. Is it possible that he just felt exhausted from having done all of the emotional work of getting your DD to sleep and it just wasn't the right time to bring up your feelings about it? Would it work better if you discuss it during the day instead? Perhaps he was feeling defensive that she was crying during his comforting of her? Or is this part of a larger pattern as you hinted at? I do think that you need to be able to discuss and feed back to one another about how different approaches are going and to be sympathetic when a change in routine is difficult especially for the one who didn't want to make a change, without perceiving it as a personal attack or a manipulative attempt on your part to get him to change tack. It does need to be open to discussion.
I think in the current situation it sounds like changes are happening so it's worth persevering a little longer, but agree a timescale at which you'll reassess. Perhaps after day 5.
I honestly don't think it's worth trying to work out the benefits/harms of sleep training vs not - realistically a behaviour change at bedtime is difficult and likely to involve crying when you're pushing it forward from when it would naturally occur - like ANY behaviour change, which you would not stop simply because a child cried even if you found this difficult to cope with. Add to that it's happening at a time a child is tired as well and they will cry about it - it's a natural and okay response, if difficult to listen to. OTOH of course it doesn't harm them not to be sleep trained, this is absolutely a subjective issue which means that it is not a debate to be won with facts or persuasion but simply a discussion to be had between nobody else except you and your DH - you've given him the respect and space to try out his method, and he needs to give you the respect and space to express your opinion on it without feeling attacked or that you're telling him he has to stop. You ought to then both be able to agree on a route forwards and a plan of action and it's okay whichever way you decide to go, but you should both be happy with/accepting of the proposed solution even if it's not what you'd do in an ideal world.
Hope that makes sense - good luck.