I wouldn't say I'm more knowledgeable than you about it, but I do have a slightly different experience of the mental capacity act. Definitely outing as I've posted about this under a different username, but to be fair I didn't actually mean to name change for this thread, but I support a young adult with a working diagnosis of autism and 15 years suffering from anorexia.
The mental capacity act, to my understanding, probably the same as yours, is that it is there to treat people when there's a suspicion that they don't understand, retain, weigh-up, and communicate information about their decisions that could harm them or other people without intervention. It is there for the purpose of treating someone, and not withholding that treatment.
It is woefully underutilised in some circumstances, like with my YP with anorexia who says conflicting things like they don't want to die, they're terrified of dying, but can't bring themselves to eat, eating feels like they're punishing themselves, they don't deserve happiness, they don't want to gain weight even though gaining weight is the only way to ensure that they stay alive. We've had 3 successful "sections" where once sectioned, nothing was enforced, we had a visit from a dietician with a flimsy name like Rainbow who dropped off an information sheet about nutritional values of foods, no access to a specialist bed in an ED service and stuck on a gastro ward, doctors who don't understand waterloading before weigh ins, or hiding things in pockets, and accept a couple of grams of "increased weight" to be acceptable to remove the section and recommend a voluntary stay where the YP has just self discharged with no post-discharge mental health support which has lead to the loathing of the people around them and then an increase in restrictive behaviours including eating. We've had clinical staff saying things that they shouldn't say to someone with an ED, like "you look beautiful/good/healthy today" which reinforces that their restricted eating is working, while under sections. A complete lack of trauma informed care.
In other cases, it just isn't utilised at all, when it absolutely should be, such as somebody being in crisis, due to a lack of understanding. This has happened with our YP with anorexia, where we've pleaded and begged for a mental capacity assessment, because it's just not logical when you're collapsing from a few steps and your bones feel like they're on fire and your heart is at risk of stopping from low phosphate to not take remedial action, but we're just told they're an adult with capacity and have to respect these are the choices they're making.
In other cases, it's used and misused. This has happened with us with our YP too, because they're not actively trying to commit suicide (is there a nicer way to say this because I feel like the word commit is still stigmatising? killing themselves also seems a bit like it's an active choice and not significant neurological distress), they're just heading to a slightly more accellerated death than your average Joe because they're choosing to not eat. We've been denied assessments and denied healthcare because of this even though we had to source our own wheelchair because of the genuine fear that even 50 steps in a day would put too much pressure on their organs.
The crux of it is they don't seem to assess capacity based on individual decisions, but feel that if a person has capacity to make a sensible decision about one thing then they have capacity to make decisions about every other thing even if that includes risks to their own health.
I'm a huge advocate for autonomy, but I just can not abide the misuse of the mental capacity act at all where if someone genuinely needs intervention rather than autonomy they're not given it, and if someone autonomously chooses to engage with services, they're refused it on the basis of behaviour modification.