I'll try to answer these the best I can be sure to also read our rules concerning highways in the FAQs wurm.mwmgamers.com/faqs.html
as that lays out requirements.
Payment is 1s per 20tile length (40 tiles total). To receive payment the road must conform to the rules outlines in the FAQs. There is also a link to our official highways map to give you an idea of where we want official highways to be run. You also don't need to worry about sanding we do that to note in our records the highway has been inspected by a GM, payment has been issued, and completed/accepted.
Other highways built outside of the official highway network may be built by players but the value of the highway needs to go through an evaluation and determination process (done by me) to be eligible for payment. If you are building a highway that is not part of the official planned network and don't care about payment it is not required to have approval in advance (slope rules would still apply though). As a guide to ensure acceptance it should connect multiple deeds or strategic geographical features in a area to boost the travel ability of the community in that region.
The exact path of the planned official highways does not need to be perfectly lined up to the map as they don't necessarily take into account the terrain and were well painted by hand. It is not required but to be safe giving me an idea of what you'll be doing in advance is beneficial and appreciated so you know it lines up with our goals and have the highest chance of acceptance.
As for bridges on highways we try to avoid them if at all possible unless there just is no other choice to make the road happen. We do not have a set payment for bridges as there are too many variables depending on length and type of bridge so it will be handled on a case by case basis.
Guard towers up until now have not been included in our work bounty program. I do see value in them and wouldn't mind seeing some. I would say to be eligible for payment they must be along an already completed official numbered highway (and have a 1 tile space between the edge of the highway and the tower). No pre-approval of any kind would be required though I would say if you are placing several ensure you are placing them at optimal distances aka the closest to each other the game permits as referenced on the Wurmpedia https://www.wurmpedia.com/index.php/Guard_tower
. As for quality this is new territory for us so off the top of my head I'd say my biggest concern here is decay rather than number of guards so despite that I'd go with 50ql+ and 5s for payment. Towers built with higher quality I would even consider a bonus to the payment to ensure less maintenance is required. I may even consider a program in the future for those helping with repairing and improving them. IF
the tower is protected by a deed I might provide consideration for lower quality towers since the decay factor would be negated like say 30ql and a lesser payment.
Hope that answers all your questions if something isn't clear post a reply and I'll do my best to address it.