It is clear to me that being on a diet for a few days or walking a bit more would do absolutely nothing about her weight.
This is the lie that she wants her audience to believe.
If she were to reduce her caloric intake to <1200 calories a day, do daily exercises for 30-60 minutes (like stretching, lifting a dumbell, walking short distances, etc., nothing overly strenuous). Then she could lose a lot of weight really easily.
Amber's biggest issue isn't the fact that she's nearing immobility, it's that she is in complete denial about the amount of calories she consumes every day. I think that this denial borders on psychosis at times. She has to eat at
least 5000 calories per day to sustain that weight (assuming she's 600lbs and 5'2 like you suggest). That's the same as eating 120 Chicken McNuggets per day (the whole nugget, not the weird skin-only thing that she does). At her size, her BMR (basal metabolic rate) is probably around 3000 calories, if not more, per day.
If she ate 1200 calories per day for a month, she'd have consumed 36000 total. If she didn't do
any exercises at all, didn't even move a muscle, she'd still have burnt 90000 calories in the month. After subtracting the burnt calories by the consumed calories, we're left with 54000 calories, or 15lbs. If she were moving more and doing daily exercises, she'd lose even more.
What is her aversion to any treatment whatsoever of the lymphedema?
This would break the delusional fantasy, which is the most horrific thing imaginable to someone like her. For any treatment to be successful, even partially, she'd have to actually confront her eating habits in an honest way, and that's impossible for her.
I don't think Amber wants to get "worse," I think that she gets worse and worse because if she actually wanted to get better than she'd have to admit there was a problem to begin with. She'd have to admit to herself that eating sodium-rich, caloric, greasy foods is bad for her. She'd have to admit that she eats in excess of 5000 calories per day. She doesn't want to do that.