He has not released his birds yet
That is good, but only half of the consideration when it comes to determining whether they will settle to your loft and not fly back to your grandfather's. Age is the other factor. Do you know how old they are? They really shouldn't be much more than 2 months of age if you want a good shot at settling them to your loft. Otherwise, if they are much older, despite not having flown in your grandfather's loft, they may have settled to that location in any event.
I'm a little worried about their long lifespan, given I'm going to high school next year and will only have four years left at my house
That's a smart thing to consider. I think most homers raised and kept well can live far past 4 years. Many make it to the double digits. They also reproduce rapidly if birth control methods (replacing eggs with fake ones, until the birds give up laying) are not implemented, which is a consideration. So, if you don't think you can find a home for your birds when you move to college, perhaps it's best to hold off on this hobby, unless you feel comfortable with simply eating them all before you go to college (sounds gruesome, and most fanciers can't stomach that, but it's more humane than letting them go for them to suffer in the wild).
we do have birds of prey around here, so maybe the population will balance
Birds of prey are everywhere, and affect all of us. You will incur losses due to birds of prey, as well as losses that are inexplicable. With both allowing your birds to breed, and not allowing your birds to breed, you can control this.
Is it profitable to sell squabs?
With the proper networking (being on here, facebook, and connected to local clubs and such), I think you will have no problem finding homes for squab you may not want, especially with solid white homers, but I will add I do not think it is profitable at all. I don't think you should expect to sell squeakers for much more than $10 a piece, which leaves little to no profit once you factor in your own costs of feed and such. You may even find yourself having to give them away. Unless you know of folks who want and need squabs, I would recommend implementing birth control methods when you think you have too many birds, rather than plan on finding homes for squabs you don't have room for.
With access outside, could they die off easily unless I raise their young to replace them?
If you start your loft properly, keep it clean and not over crowded, and also have at least 6-12 birds that can fly together, I think you will find that your losses will be somewhat offset by breeding. I recommend keeping a loft that can fit 3x as many birds as you want to have, so you have flexibility in ensuring you can breed a good amount to not have too low of numbers from losses. If you fly your birds, you will have losses at some point, and you may even experiences losses that seem inexplicable and unfair. It's just part of owning and flying pigeons.
I could maybe get 1 breeding pair and control the population like that?
I wouldn't recommend that. I have found that it is easier to settle birds and train birds to home to your loft in squeaker aged groups (1-2 month old birds) in numbers not much less than 12. They learn better in larger groups. With one breeding pair, you'll only get two squabs per clutch, so your training groups will be small and staggered. You will also not be able to fly the breeding pair since they will not likely re-home to your loft, and if they do, it will take 3-4 rounds of raising squabs. I think your best bet is to get 12-20 squeakers from your grandpa, settle and train them to your loft, and wait 6-9 months for them to start breeding. This way if you lose even half the birds in the process (which is quite possible), you'll still be left with 6-10 birds that are settled to your loft and will start breeding at some point.