Once you leave Fort St. John behind, your cell towers are almost all Bell Mobility Canada until you hit the AlCan border. Google Canadian cell tower map, and it will show you roughly where they are up there.
If your US cell phone company has a roaming reciprocity agreement with Bell or Telus, your phone should work through that stretch if there's a tower nearby. We each have a dual SIM phone, and we have an AT&T (Rogers) SIM card in her phone, and a T-Mobile (Bell/Telus) SIM card in mine. Along with our Rogers Canadian SIM cards. We're covered. ;)
We got caught up there 2 years ago without cell service until we hit Whitehorse, where we did the same thing you did, and bought a Bell based SIM card for one of the phones.
Honestly, there aren't that many cell towers up there anyway, except as you described, in/near the bigger towns.
The rest of your observations are true.
