For an item, what is the best way to find the exact line on the parent order to get the qty from? I don't want to use item id in case there is ever an example that there are 2 of the same items on the order. I thought id was it, but that doesn't seem to be working and seems hit or miss

I think most cases I've seen, a custom column populated with a unique value is added to be able to reliably track lines across transactions. If someone suddenly tells me there is a uniquekey that actually "travels" I'm all ears.