The menu says that it comes with lettuce and tomato unless otherwise stated and next to the chicken burger it says feta and rocket...... so otherwise stated.
Do they offer baked potatoes on the menu? Or did you ask for one off menu and then expect an oven baked one? Because it takes quite a while to bake a potato in the oven, so unless they're offered on the menu as standard, common sense dictates that it'd be microwaved. If it was hard in the middle then that's an issue that should be raised.
Having an ED can't be easy especially when navigating eating out, but as your husband says this happens all the time, it seems like you're taking it out on the restaurants you visit and worse, your family are also having to deal with it.
I mean this as kindly as possible, but this is your issue and not the responsibility of the restaurant to deal with, from what you've said, you got what you ordered, as stated on the menu, but because you're unhappy around food, regardless of that you complained anyway and got a reduction on your meal - it doesn't always mean you're right if you get a discount, it can mean the restaurant is trying to avoid a scene (which other customers are going to then complain to the restaurant about) and a trashing online with a review.
The same goes for sympathising with you, even in a clear case where a customer is being unreasonable, telling them so is just not done in most circumstances because it's seen as rude and poor customer service, and doesn't resolve the situation, it escalates it.
Listening and sympathising is one of the keys to good service, even if the customer is plainly being unreasonable - and that's what you got here, as well as getting what you ordered.
You misunderstood the menu, it happens, and you were compensated for that, even though the restaurant as far as I can see, didn't do anything wrong here (except maybe the hard jacket potato).