UKABC the research about dc crying/discipline says in a nutshell that to teach a child how to self soothe and how to be empathetic to others, you soothe them. The soothing them both calms them at the time and builds connection between them which makes them want to listen to you , and makes them happy, and also teaches them how soothing is done. So if they are crying because they want something they can't have, you don't give in but you do at the same time as saying "no" sympathise with them, and soothe.
I think allowing adopted children space to deal with their own problems is a good idea but when the child is older - not at 2 years. And the way to help an adopted child to be able to do this is to teach them skills - soothing but also other skills which children can learn gradually - like, judgement, empathy, self control - it is a gradual learning curve.
I do think that limiting choices endlessly and too much control is disastrous for adopted children. But the above isn't doing that - and applies to both adopted and non adopted children.
Sleep training - from a quick look, there is research saying that sleep training (as distinct from CIO which is the more harsher and outdated notion I think?) or holding a child til they sleep both are fine if done well attuned to the child's needs.
If you have a look at ahaparenting.com or parentingscience both set out the research on all this. Obviously things change over time but both these sites state the current state of research, and there is a lot of research generally saying the same thing. Both appear to be comprehensive too and so will set out the dissenting research.
Penelope Leach was saying a lot of it back in the 70s!
The emotionally intelligent adults I know either learned it as kids from their parents or as adults from books. Kids can't learn to get rid of their demons on their own as children, and having to do it as an adult instead, later, is a lot more painful. in fact, if you help dc well, they won't have demons to get rid of.