Am I right in assuming that the initial point of:
- a floor texture is always grid 0,0?

Yes

- a wall texture is always wall bottom,start point?

More precisely, the bottom in question is elevation 0, which is important on a house with multiple levels.

So while a floor texture is fixed to the plan grid, a wall texture is relative to the wall?

As wall can have any angle, it couldn't start from a fixed point.

I'm tempted to add scale factor and x/y offsets parameters in a coming version of the texture dialog box to let the user adjust existing textures in cases like raczek's one, but also for furniture that includes some texture coordinates, but fear it might not always be correctly understood. [:(]

And sky texture bottom starts due right in the plan, on horizon; and stretches completely 360 deg in the horizontal plane?

Yes

Whereas the top of the image compresses in the top of a sphere/cone above plan?

Compression isn't the right word, it's more like the used projection requires that the image is equirectangular.