OP, I'm with you on all except the popcorn/chips.
I think if you have mixed ages you need to be more flexible about outings. Yes, some things will always appeal more to one child than another and the others just have to suck it up. But some things are just sooooo boring for the other children that its simply not fair to impose it on them.
All the modern stuff - she was out of order. I suspect she did it because she didn't feel as though she was the only one suckered into buying all the modern gadgets for her DCs. If she wants to buy it, then fine, but to encourage your DC to pressure you was out of order. My DC get spoilt with some things, but not with others. I choose what I will get them carefully, balancing out wanting to get them nice things, but still wanting them to appreciate that these things cost money.
Generally, I would say I veer towards your style of parenting, but perhaps not quite as strict. I will tell my DC that something 'might' happen, I don't always buy them treats when we go out, I don't buy them all of the latest gadgets (but might be one or two)
But with regard to the popcorn/chips - sorry, but at 8 (DS1 is 7 so similar aged) I would have got him what he wanted. But only the 1 thing. As he pointed out in one of the battles - 'why do YOU get to choose ALL the time?' And it's true. Why should I get to choose all the time? What is the difference between popcorn and chips as a treat? I can make the choice that they only get one, but he should get to choose what that one is. Especially if I had TOLD him he could choose a snack. Just because you had bought him something, I wouldn't impose it on him. (But I would have made it clear to you that I had already promised him that he could choose his treat, and apologise) - food wastage be damned, that to me would be a dreadful reason to go back on giving him his choice.