Re tie downs
As you tie down the load, yes technically you are applying an extra force to the floor but no more mass apart from the mass of the strops.
Yes you need to take into account the mass of the spreaders too.
This is similar to one of the EASA/CAA exam questions that came up a few months ago where it asked for the floor loading but included the mass of the battens in the question. It was not a load spreading question as such but a floor loading question and the dimensions, mass are the same.