It would not be wise to take a leaking inflatable sleeping pad on any trip where you actually need a sleeping pad, and cannot hike out and replace it easily.
The pad is already leaking. You have to weigh the probability and consequences of the leak getting worse, maybe much worse, on your Alaska trip vs the cost of replacing the pad now.
Hypothermia at 50 degrees or so is unlikely if you have warm clothes, a change of warm clothes, rain gear, a sleeping bag rated for significantly colder than 50 degrees F and a tent. I doubt you will get hypothermia even with a completely deflated pad. And you can always put on more clothes, or spread some clothes under you in your bag. The only caveat about hypothermia at night at 50 degrees F or so is that if it is likely to rain continuously for several days, but that is a different question. There is another caveat, by "below 50 degrees" you didn't mean below 40 degrees, did you?
As for comfort, you will be much more uncomfortable if your inflatable pad seriously deflates than if you used a foam pad. Also, if you are really uncomfortable with the foam pad, get tireder than you normally do; that often guarantees a sound sleep on anything!
So, unless you are on an extremely tight budget, I recommend that you replace the leaking pad.
The answer to your last sentence is yes, but again, bear in mind that 50 degrees F or so is not really cold. I've used an air mattress when the water bottle outside the tent froze solid, and I was perfectly warm.
(Not completely frivolous aside, see the movie Alpha for inspiration. 20,000 years ago people didn't have inflatable or foam pads, and managed to sleep well. We should be able to do anything they could do!)