I was able to fix the entirety of the walkmesh issues by increasing the tile to tile width beyond the size of what a character's combat box size is. It seems the character size for determining if you can quickly walk past something is much larger than it's actual size, both in terms of model bounds, and what it claims to take up in the 2da files. Pathfinding seems to use CREPERSPACE rather than the smaller number PERSPACE, and does not rely at all on the SIZECATEGORY entry. I suggest making sure that any open edge (tile boundary edges) which a character can travel past, are at least the CREPERSPACE value (in meters) long. Removing ones that were shorter, or otherwise combining two edges to make one longer than CREPERSPACE, cleared up the issue entirely. I was able to leave some of the tiles with 45 degree splits up the middle without causing any further issues.
Edit: Just to clarify a bit... If your character is 2m wide and you have 1m wide edge on one tile which a character can walk over, then he cannot fit through. If you combine that 1m edge with another 1m edge on an adjacent tile, he still cannot walk through, because it doesn't matter the whole size of the opening, but rather the opening on the individual tile. Pathfinding told him to cross at the short spot, but when he got there, the calculation told him it is too small.
I find this is about the same for much larger creatures, like dragons and demons. They simply stop walking a lot, even if the overall opening is obviously large enough to walk through.
As mentioned, a way to get past the issue on existing tilesets is to simply slow walk them over the threshold. This usually works, but can be maddening, as the character may decide on a path that goes a mile around what you are trying to have him walk, sudden turn on you, and walk away.
