I wouldn't worry too much about the Insight being able to get through the mountains. Even without the IMA, you can always downshift to fourth or third if need be. You might have a little extra noise, but the car is so light, it really shouldn't need more power.
Now, I'm not talking from experience here, but I seem to recall reading a thread somewhere of a guy driving with a completely non-functional battery pack and still getting good mileage out of the car. I don't recall any complaints about the performance either.
Even if your batteries are indeed toasted, I'm still jealous.
JMac
Edit: Didn't see Jamesqf's post. Definitely explains it better than I.