I've asked all these questions myself to others recently, so here's what I've gathered. All are encouraged to correct me if I'm off here.
qbradq wrote:1. Should a fence also be used?
Yes, having a fence and a tree wall means they have to switch their equipped item, and fences are much, much easier to build than tree walls so why not, the more layers of wall (whether tree or fence) the better. Also tree walls don't splash so you can build them right next to each other and right next to fences.
qbradq wrote:2. If yes to 1, should the trees be inside or outside of a fence?
Typically they're the outer-most layer, maybe because you can't repair trees from the inside but you can repair fences from the inside, so fences being on the inner layer of a fence sandwich is preferable. Also, maybe visibility reasons so that the trees don't make it hard to see while you're inside (though the difference of a few tiles is small). Also you can keep adding more layers of tree walls right on top of the previous layers in the future, which makes more sense to expand outwards than inwards.
qbradq wrote:3. Should the trees be within the claim?
Yes, otherwise a 5 alt could grief you.
qbradq wrote:4. Should their be a 6-tile border outside of the trees within the claim?
No, the 6-tile border is to prevent splash damage. Trees do not take splash damage, but they do not stop splash damage either. So make sure that the fences behind the trees are 6-tiles from the border but where the trees are as far as this goes doesn't matter. I like to put makeshift fence on the very edge of the claim (doesn't take splash either) and then start doing treewalls behind that til you get sick of it and then put the start of your real fences 6 tiles in.
qbradq wrote:5. Will cutting a tree / removing a stump on a claim cause brazers to open fire?
Yes, I believe it counts as stealing. You may have to be on the claim to trigger the brazier fire, even if you're cutting a tree on the claim, which means don't put trees on the very edge of the claim.
qbradq wrote:6. Any solution to gates other than using many spaced 6 tiles apart, or with split-rail in between?
This is tricky and requires a bit of creativity. One solution is to fully encircle your claim except for the entrance with trees, then make a tunnel of trees at the entrance with multiple gates with 5 tiles between them to avoid splash between the gates. That way they either have to cut through the tree wall, or go through the tree tunnel and break through multiple gates.
qbradq wrote:I would also be interested in what kind of bases you would put a tree wall around.
Depends on the player, some people put treewalls around ***** -everything-, some people never build tree walls. Just depends on your motivation level/OCD-ness/etc. If you want more defenses, layering tree wall on top of fences is a promising option. I would probably not suggest doing a tree wall unless you're also doing fences though. If you want to half-ass it, just doing a layer of stone or stone/plank would be much better than just doing trees for the effort/benefit ratio.