I understand why you feel so angry - I love my daughter too and hate the thought of her being hurt in any way.
However, you can't fix this for her. Life involves some challenges and some suffering - you can't make it go away. She has to navigate friendship issues herself. All you can do is listen, sympathise and advise.
There aren't that many concrete incidents in your post - that's fine, I assume there is lots you left out for brevityz for you to feel so strongly. To address the ones you mentioned.
A) the smell in the classtoom (were they implying she farted, or that she had BO). Either way, it is a fairly common thing for this age to banter about - maybe boys more than girls, but even so. Yes it can be hurtful and embarrassing to a sensitive child, and the girls were wrong to go it, but try not to get it out of proportion - it's the sort of thing everyone else in the class would have forgotten by breaktine
B) the backpack - frankly it was, as you say, a weird thing for your dad to offer. Ideally the other girl would say no, but perhaps she genuinely felt your dd was happy to carry it as she had asked - I might carry a friend's bag if she was slighter than me, or feeling tired - if I offered, I wouldn't consider it wrong of her to accept.
C) homework - this is totally normal - kids help each other out with homework all the time. Kind of your dad to offer, not wrong of the girl to accept (I mean, yes, technically cheating, but not wrong in a bullying sense).
D) the turning round with a mean face and the comment - not nice obviously. The face bit could be subjective but the comment does sound a bit cruel. Try not to encourage your dd to see it as the end of the world though - it's not.
Above all try to model resilience and help her keep things in proportion, for her own sake. It does sound like seeking out new friendships would be good, but getting lost in misery and rage about what is happening currently won't help anyone.
Having said that, I do get fierce when I think someone is remotely mean to my daughter, so I feel your pain!
But you owe it to her to let her grow and learn through this.