It will be fine to mix the icecubes together. The only time that's not ok is when the breastmilk you're mixing is at different temperatures.
I'm not sure why but for some reason expressing or feeding in the wee small hours increases your milk flow so maybe try getting up a wee bit earlier and expressing then.
I also found that my flow was always better when I expressed off one side and fed off the other. It is very easy to do, just make sure your baby is latched properly and propped up on a pillow so you're not bending your back and then start expressing from the other side.
The other (IMO most important) thing is to relax and don't stress about it. Nothing killed my milk flow when expressing like stressing that I wasn't going to have enough milk. It isn't the end of the world if you buy a single pack of formula to send over as a back up plan but it sounds like you've got a good wee stash there and if you feed Emily at Nana's house just before you leave that should help the milk supply last too.
Good luck and enjoy your night off.