I'm posting this as someone with an ED, rather than as a parent. My ED disorder developed when I was an adult, so not necessarily similar to when teens develop EDs.
Eating disorders are very rarely solely about weight, food and wanting to be thin, although that is how they present and those are the visible symptoms. They are very much about control and their pathology is very complicated. I consider myself a very rational, logical person in many respects, but someone with an eating disorder does not have rational thought processes around food and eating and it can be hard for other people to understand the mindset of someone with an ED.
I think some of the habits described on this thread are concerning and I can understand that knowing when and how to intervene is difficult. Some things that might be helpful are avoiding conversations about food and eating during meal times, avoiding labelling foods as good or bad, or healthy or unhealthy and avoiding talk of body shape. If there are scales in your bathroom, I would suggest removing them - and if that causes upset that would be a red flag to me.
If you are concerned about lack of calories you could try adding things like adding more olive oil or butter to dishes, or encouraging food like nut butters which are nutrient dense. I am in a strange place in terms of recovery, but one of my issues is around volume of food - so I'd rather eat a slice of toast and nut butter than a plate that has more food on it but with the same calorific value. I also have lots of milky drinks when I am trying to increase calories as my eating disorder is a lot less anxious about calories that I drink compared to calories I eat. As I said - they are not rational thought processes.
Beat eating disorders charity is a great resource for carers and their guidance is really comprehensive.
Seeking help sooner rather than later always helps with ED treatment, but is not always easy to access and requires specialist intervention rather than just general CAMHS. In adult cases, many GPs are overly fixated on weight as an indicator of an ED which can be a barrier to early intervention - I am unsure if this is the case with teens, but I would suggest treading carefully as telling someone with an ED that they essentially weigh too much to access help can be damaging. This is a real challenge in terms of early intervention.
This is a bit of a ramble so I hope some of it is useful. I think my final thought is that when someone has an ED, the fear of food, or certain foods, and the anxiety around them is so real and intense that trying to force someone to eat is never a battle that you will win - it just causes distress for all parties. Even if it seems someone is being stubborn or difficult, it is the eating disorder causing that behaviour, rather than the individual themselves