For that saving, I would buy it on line too.
If you decide to print at home, you will be given the pdf?, sent to your email. If so, then even if you lose the tix and you have access to a PC, you can open your email and re-print the tickets again. I wouldn't worry too much about losing this on the way. We've certainly way-laid stuff in bags and its been easier to re-print rather than upend bags looking for papers. And you're staying at Disney....they will be more than happy to help you out.
For your Will-Call tix, I assume that there will be the option to pick up at Guest Relations; which at IoA, from a rather faulty memory, I believe is on the right hand side of the entry gates where the queue starts. I would definitely check this out...if not on-line on the UA site; then post a thread on the UA section of the boards. Pretty sure someone will be able to help there.
Alamo is the "budget arm" of National. They are on the WDW property - but it is a VERY big plot of land. I understand that they are happy to come and pick you up to their premise if you're calling from a Disney resort. Allow about 3 - 4 hours for when you get picked up to getting the car.

I haven't checked the transport board in a long time...but that seemed to be the standard complaint on the board. It probably takes much less time...but if you plan for the worst, then you'll be happier with less time. Alamo has a different website to National.
If you go on the Alamo website, you will have the option for different pick up and drop off locations. It might be worth considering dropping off at MCO and using the car instead of ME.
Another thing about car hire...have you looked at prices for car hire in Honolulu? It may be more value for money if you have more days; rather than less.