No, the base stealth radius is the same for every player, no matter what level. But, there are some things that increase the radius, e.g., the third passive from medium armor.2) Can the radius of stealth decrease between different level players? Level 10 vs V14 for example
No, you can (and imo should) leave the siege weapon while it reloads. That way, you can react much better to incomming siege fire from your enemies or a simple player attack. Once it's reloaded (as noticed by the reload animation), get back on it and take your shoot.3) When deploying a siege weapon, does leaving the weapon decrease its damage? (Instead of staying in aiming mode)
Yes, for example. You will have noticed that every keep or outpost that has its outer walls below 50 % at some place will show in a different way on the map. Also, fights are displayed on the map by crossed swords. Other than that, pay attention to the zone chat, although many things don't enter zone chat due to spies.4) On the default pvp map, how do you tell where the attacks are? Just by looking at the siege weapons deployed?
There has to be a continous route from the place you're at to your destination. Furthermore, you cannot travel to keeps or outposts, and cannot respawn there either, that show on the map as being attacked (outer walls below 50 %). And there is something with the ressources, if I remember correctly. (If you win a keep, you need to own all ressources before you can use the transitus.)6) Why can't you quick travel to some waypoints that the map says your faction owns?
That depends on many things. You can allways use siege weapons. All men are equal behind a ballista. You can stick with a larger group. Just try for yourself! You should definitely not try to hide and trap other players as you will most likely just be a snack for them.
7) Does a level 10 contribute anything to pvp? Can they even kill a Veteran rank?
Edit wants to thank you for a double post. Always refreshing to spend some time writing an answer just to discover that all those answers were already given in the other copy of this thread.