Hi Firepile. I also have a 10 month old who has dropped down the charts. She was tracking at 90th centile, but at last official weigh in had dropped to 50th, and over a month on from that she still hasn't gained a thing (by my own scales), which makes her lighter now than she was about 4 months ago. Obviously she isn't off the charts like your ds, but she is around 75th centile for height too, so there's more body to spread her weight around. I wouldn't describe her as at all thin, so I do think the charts are off on this, and we shouldn't be beholden to them.
My DD has been cruising since just over 7 months, and doesn't like to keep still at all (won't even sit in her pushchair!), and I put her failure to gain weight down to this. I imagine this may be the case for your ds too, since you say he is very active. Since the charts are based on averages, they are bound to be misleading for babies who become mobile much earlier than average.
I am also still bf (DD refuses to drink more than a couple of sips of formula, but she does have to have a quite hideous hypoallergenic one, as she is cows milk protein intolerant). DD also often refuses to eat solids. We are doing BLW as she won't let me spoon feed her at all. She's quite capable of feeding herself, but just chooses to throw the food on the floor instead most of the time.
Anyway, the point of all this is that no, I don't think you are a freak for not wanting to feed your ds chocolate biscuits, and am frankly horrified that anyone in the medical profession would even suggest it. Maybe an odd biscuit as a treat when you are having one yourself wouldn't be the end of the world, but I certainly wouldn't intentionally go out of my way to give them. I would have thought that that would only develop a sweet tooth and make it even harder to get him to eat good healthy food.
Your menu, with perhaps some oily fish thrown in and a few things cooked in olive oil (diced potatoes done this way and mixed with chopped egg and baked beans might be a hit) sounds just great.
As for the 'what if' question, I'm a great believer in addressing that issue if and when you get there.