It seems any boar can spawn as the 1 skull so it may come down to just killing whichever ones you can get to over and over until it spawns.
My go to spot is in G8 where the player marker is on the linked screenshot of the map. Slightly north of center. Though this may be one of the built over spots you mentioned. I’ve seen this one up even with buildings right beside him but if the entire area is encased then that’s definitely a problem.
If you head to the npc camp at the pocket, in the cliff to the left of the camp as you approach it is a guaranteed 1 skull boar almost hidden away - that one works
The boar will spawn from the ramp from the Savana to the Tundra biome all the way to the Camp at the end of the colossi statues.
And they will walk around quite a bit. And at the point Freya tells you she is too far, she still follow you for some time before resetting.
I have found the boar even over the stone mesas lately, which makes me think they changed the spawn area because of people complaining they could not find it.
They moved the spawn to 100% on the area above the cave tunnel to the southern regions. If you ask her, Freya will say “On the cliff overlooking the sands”, or something along that line.
I have found a boss boar just below her hovel, and in other places where boars spawn.
The biggest issue is that players build over the spawns. There’s not a lot to be done about that if they aren’t willing to move.
As for the other kills;
They did add a 100% corrupted wolf spawn to the small pack on the west side of the aqueduct below Lian. That’s in case the white wolf is a puppy, as I have found more than once.
There are two elk spawns, one of which is under the aqueduct under the dam.