1. Is it true that there are NO services along both the road to Harbour Breton (NL 360) and the road to Burgeo (NL 480)?
When I was last there in 2011 and photographed the fuel warning signs for each, the NL 360 sign said no services for 129km, but there are two communities south of there before you get to Harbour Breton at ~km200. NL 480's sign said no services for 147km, in Burgeo near the highway's south end.
3. Why are the numbers the way they are? It seems like each subregion of The Rock has a consecutive digit to the nearest ten, i.e., SW/W NL has routes in the 400's, like 420, 430 and so on and they get lower as you go east. they also like to go by 10's.
The routes are indeed clustered by region, with the Avalon region in the SE getting the xx and 1xx routes, the northwest and southwest getting the 4xx routes, and 2xx (including but not limited to the Burin Peninsula) and 3xx routes in-between. Labrador gets the 5xx routes, with 500 and 510 for parts of the Trans-Labrador Highway, as well as several branch routes from 503 to 520.
There are plenty of routes with a last digit other than zero, often branches from major routes ending in zero.