MiaMamma, if I were you I would express as much as possible, since you have quite a lot of milk to get and not much time to do it. If it helps, I used to find that I could get much more by getting up early and expressing before my first feed of the day (but she was sleeping through the night - it may be different if she's not).
If you don't know this already, the cheapest bm freezer bags are from Boots.
I weighed dd's nappies to get a rough idea of how much she was drinking (obviously not 100% accurate because of liquid lost in sweat and breathing) and I reckoned it was around 21oz a day. I also read somewhere that 20oz is about average at around 3-4 months. Of course, you need a bit extra as some ends up getting thrown away if you overestimate how much you need for a feed. I was advised to freeze in different amounts (5oz, 2oz etc) so you have a choice how much to give at a feed.
As I understand it (and I don't really understand the science behind this, so someone who knows better may correct me) the problem with travelling comes because it's not safe to refreeze already-defrosted milk, so if the milk defrosts on the journey you can't freeze it again. You may be lucky with a cold bag and lots of ice blocks on a 10 hour journey, but I tried this last year on a 5-hour drive up from the south coast and my milk defrosted and I had to throw away a week's worth of expressing. Still, as NQC says, an aeroplane's hold is really really cold so you may well be lucky!
If you are flying somewhere with rules about importing foodstuffs, maybe you can confirm in advance that it's ebm and that it doesn't count as dairy - you don't want to turn up for your flight and have them refuse to carry it.
As to whether you should introduce her to formula in advance, I would say you have to decide based on how likely it is that you will manage to get away without giving her any, how concerned you are to keep things smooth and easy for your mum, and how concerned you are about maintaining her 100% bf status. Eg. if you think your mum probably will have to give her formula at some point, you can make things smoother for your mum by introducing it to her first (so dd doesn't have to deal with strange person and strange food all at once). On the other hand if you think you can probably get away without it, then why lose her 100% bf state just on the offchance that something might go wrong. After all, if there is a disaster (eg the airline loses your milk!) and you have to give formula, or you miscalculate and run out unexpectedly, she'll be fine in the end -either she'll drink the formula, or she'll just bf more when you're around - she won't starve (though might give your mum a hard time if she's not happy about it, or keep you up all night feeding!)
As HM says, bm has really good keeping and antibacterial qualities - better than formula or cow's milk. 'The Womanly Art of Breastfeeding' (by the La Leche League) makes out you can be amazingly relaxed, eg. not sterilise your breastpump every time you use it. If you can express during the day I'd have thought the milk would be all right in an an insulated bag or thermos stuffed into your rucksack, specially if your mum then gives it to her the next day. I don't know what kind of pump you use, but I personally only used a hand pump and found it took ages, which I didn;t mind when I was expressing at home, but when I was expressing at work it was a pain in the neck to spend 30-40 mins locked in my office able to only type one-handed! So a battery pump might be the best kind.
What country is it by the way? What is the weather like there, and the attitude to breastfeeding?
Hope some of this helps.... BTW I think it's fab that you are going for this. Lots & lots of luck.