It had crossed my mind to take up the tongue and groove and put down chipboard flooring even before i noticed the bowing - there are a couple of places with odd holes and roughly removed/replaced boards. I need to do something like that under the toilet where the floor boards are a bit rank. Is 18mm wbp ply thick enough in place of the floor boards and is it better than flooring chipboard.
Mate, the floor bords will not be the main problem. In most cases the joists may also have warped or sunk etc due to movement, moisture and movement as expected over time so laifting floor board and sheeting with ply is not neccasseraly the best solution. If the prep work is just for Amtico or Karndean etc, then you realy just need to make the existing floor level. You can acheive this by sanding down or filling and dips etc with a screed and then sheet over the area with 6mm ply you were inttending to use. Just remeber to prime before screeding and feather finnish any joints between the board of ply.
With the right products this will be a faster way to do than ripping up all floorboards and then sheeting with 18mm ply. 18mm ply is not a cheap option and after screwing you will prob still find that the floor is not flat. Some old floor boards may need replacing but I wouldnt replace them all if it was me.
I have experience in this type of flooring and have been employed in the industry and the ways I have mentioned were used with success.
Design vinyl does not require the absolute stabilty that ceramic and stome do as it will not break and crack the way ceramics will.