Routing signals in priority

First, the router will give timing critical signals the highest priority, so in essence they are routed first. So your timing constraints already dictate this. (It's not a straightforward "route this first" assignmnet, but usually you want this for timing, and the router is already doing it for timing.) If it's not for timing, what's the benefit of having something routed first?

You can go to assignments -> Settings -> Back-annotate and do it to the routing level. This will create an .rcf which has all the routes. You can use this, as it's somewhat readable, if you want to preserve specific routes. Just keep what you want and rip the rest out.