I'll try and answer them. Sorry for the brevity.
Not all ground air is suitable for packs. Sometimes it's only good for engine starting. Sometimes there are other restrictions. Sometimes someone else got the equipment first.
There probably is no agreement but traffic from DFW, Mexico on those sorts of tracks out over the pacific is probably minimal. Getting levels wouldn't be a huge drama. Besides that, the 744 is actually pretty good if you get 'caught low'. As long as you're prepared to let the CI slow you down, you can end up 6000' below optimum after 5-6 hours and it will cost you about a ton of fuel (I haven't flown the 744 for about three years so I'm going on memory). You will have lost 10-15 minutes over the plan but you won't drop that much fuel.
QF fuel policy isn't 15% for the whole flight. It's 15% up to a maximum figure (can't remember the 744 one). There are some other nuances as well. in short, as long as we have fuel to a 'suitable airfield' at all times during flight we're golden.