1) Can I shift it into neutral and push it out of the garage?
You have a couple of choices to move the Prius out of the garage. You cannot shift it into neutral without the 12v battery. What you can do, however is jump the vehicle from the front end. Here is a picture I acquired from this website which shows where to put your jumper lead at (the pinkie finger):
You can place the black (negative) lead on the gold colored stud/nut at the 11 o'clock position. This should get you enough juice into the system to put the car into neutral (probably not enough to start it). Once you have the car in neutral, you can remove the relay which is where the pointer finger is pointing to (already removed in the picture). Then once you remove the power, the car will stay in neutral and you can move it.
If this isn't to your liking, you'll need to lift the front of the car and roll it out of the garage. This can be accomplished using a rolling floor jack, or by placing the front tires on roller dollies.
2) Can I open the rear hatch to jump the auxiliary battery in the trunk?
I would assume you could open the rear hatch using the same method as above (using a jumper on the system as described).
3) Is it worth buying a 20ft jumper cable?
My jumper cables are about 20ft long and are very low gauge (2 gauge, I think). They will transfer a lot of juice and have always come in handy. I've had them for over 20 years and have never regretted the purchase. These will be pretty expensive to get, but worth it in the end, IMHO.
4) Should I call road service?
If all else fails, yes. It's about the last resort, but it could be about your only choice if you cannot do it yourself. This is up to you, though.