Re the tile alignment in blender (or anything really), you have to make sure your rotations are correct and your exported heightmap shares the exact same format.
Re the collisions, you just create simple convex collisions for the tiles - using automated systems or the engine.
You set the number of poligons to maximum and the number of vertex to maximum you get faily decent performance and fairly decent collision.
Definitely don’t go with complex as simple.
The heightmesh sounds like an interesting alternative, but the load of it for a tiled or tiled like world may not work.
On the other hand, RVT on world comp tiles are probably still broken - to where you need a different material set up for each tile, I’m sure.
Each component is a drawcall. So less components means better perofromance.
To a point anyway.
Generally definitely use 2x2, so its less componentns.
The rest depends on the size you need really.