+1
First of all, love the idea, expanding the trucker job would definitely improve roleplay and might even create its own small community of truckers!
Regarding your questions;
(1) I think truck owners should receive a symbolic pay increase, such as 500 dollars, to encourage people to drive their own trucks. I believe that players who want to roleplay as truckers, will invest in a truck, pay increase or not, but the symbolic pay does encourage players to engage more into the trucker community and roleplay.
(2) There could be two or one major trailer location(s), where truckers could take jobs from, which would provide them with both the route and the trailer.
(3) Prices should be based on the length of the route, as well if the trucker is driving for a players order ( Player owned business ) or an NPC order. The player order should reward truckers with 250-500 dollars extra, besides the pay based on the length of the route.
(4) Depends on how much the players earns on a general route. If the player gets 2k for driving up to Paleto from the port and back, then it should max be 15-20 seconds. but if the pay is high (3-5k) 30-45 seconds would be reasonable.
(5) I don't think a system like that is even scripted in, but if it was possible, then it should definitely be added. Maybe even damage goods when the trucker reaches over 90-100 km?
Hope you find my answers useful.