Haven't read the whole thread, but here's my two cents.
The things that you are calling minor issues, that the child should capitulate on? By expecting that capitulation, you're sending the message that actually, they ARE major issues. Your issue from what you wrote is that you don't believe they should have the right to force these minor issues. But isn't that exactly what you're doing?
There are two sides in this equation: the child, and the adult. The adult should be in charge, but the adult also has to ask themselves, how am I helping this situation by digging my heels in and making this a battle when it doesn't need to be?
Kids are unreasonable sometimes. They lack emotional maturity. hey do kick off because they were given the green plate when don't you KNOW they're supposed to have the red plate and all food tastes YUCKY off the green plate!??
What you need to do, is take a breath. The color of the plate doesn't matter, what matters is that the child eats something. So give in. Likewise with other trivial matters. Let the child assert some sort of control over his/her world - it actually leads to FEWER tantrums, (n my humble experience) when kids don't feel like every aspect of their lives is in someone else's hands.
Let the baby, to use a common phrase, have his bottle. Save your strength, your patience, and your credibility for when it REALLY matters - like being destructive or causing harm to the child themselves or others.
I say "credibility" because as much as you might think "you are the adult", if the child sees YOU kicking off over every little thing, it's going to mean a lot less when you REALLY need to put your foot down.
Good on you for coming here to ask though! The sign of a good parent/grandparent isn't that you have all the answers, it's that you care enough about the child/children to ask for help because you know you don't have all the answers 