AOE and Portal Range
I don’t know for the AoE but the portal can only be placed at the mesmer’s location, it has a range but does not need a line of sight. So if the mesmer already was at your wall and then jumped down then yes, your wall does not need to be breached.
The Mesmer portal does not work as the skill discription says it does. The portals are bi-directional and there is no dedicated entry and exit portal.
So if a Mesmer hides in a keep after capure he can place a portal inside the keep then hop over the walls place the other portal and people can enter the keep via these portals without bothering to tear down the gate or the walls.
Since both portals must be placed within 60 seconds and only remain active for about 12 seconds and have a max distance of 4000 – 5000 there’s two things to watch out once you capture a keep or once one of your walls has been briefly breached:
(a) Scan the keep for hiding Mesmers
(b) As soon as you see a group of enemies clump together in portal range for no apparent reason it’s probably the rally point for a Mesmer portal.
During fights (keeps or in the open) you should also always have a few observers at your flanks and rear to watch for the pinkish circles that are the dead giveaways of Mesmer portals.