The answers to a lot of those are "It depends." LOL
I never ever ignore tantrums any more; they're what happens when little kids have emotions that are too big for them and they need help navigating them. I'd say prepare for them, and prepare to acknowledge to her that yes, she's unhappy that she can't have so many of these treats right now but her body needs a break, and prepare to intervene to protect the other child. Then out of the moment - or maybe even pre-emptively - you can talk about things she *can* do when she's so angry and overwhelmed: have a pillow for her to punch, or a place she can bang on or stomp on the floor, and then a place for her to curl up and process and recover when the storm has passed. (And really, this is good for ALL tantrums no matter what the trigger was, not just food-based ones.
The diluted juice isn't so big a deal as straight juice or cordial; you can always gradually dilute it more and more. I remember at one point we had Josie's juice so diluted that we could pour a half-inch of juice into a half-gallon container and fill it with water so that it was *barely* pink - and that satisfied her while giving her barely any sugar at all.
For my part I'd keep treats as just that - treats. Special occasions, or maybe after-supper dessert. For mid-day snacks, I'd offer fruit or cheese or yogurt or veggies and dip - and maybe (again pre-emptively) have some yourself and invite her to share with YOU, rather than making it about the sweets/snacks. Mine are older, but right now when they get home from school they can have a snack of some sort (which usually turns out to be yogurt - I make a gallon of plain yogurt a week) and they may have something more overtly sweet after supper and *occasionally* at other times, and that seems to work out well for us. DH got a book about food garnishes when Josie was about 4 or 5 and she LOVED (and now Natalie does too) turning food into pretty things first; even now we can give them pieces of fruit and veg and make a dip from yogurt and herbs/spices and ask them to arrange a plate and they'll have fun with it, and especially at the beginning, having them involved in the food prep made a HUGE difference in getting them to eat healthy foods.
In the end, it's going to be about re-shaping your (plural
) relationship with food, and again, your DH is going to have to be on board with it. My MIL used to try to undermine our efforts to NOT give our kids junk food and went so far as to do it behind our backs and tell the girls not to tell me! (Fortunately, they also know that as small kids, we don't keep secrets from parents; that is a dangerous precedent to set.)
I'll mull this over some more while I have breakfast and see if anything else floats to the top.