unfortunately the only way to merge is to make a new quad 😓
so it involves deleting the existing quads and making a new quad (press B and select vertices)
BSP (or CSG) was just a tool I thought would making maps easier
they are not required to be used by any means 🤔
(but you can use it if it feels easier)
the problem was with them that after around 100 operations
or after certain ones the whole BSP breaks and errors start popping up
and turns out that is just the nature of dynamic BSP and there is no easy fix 😓
(that is why there were groups added for them - and groups are
BSP 'islands' that don't interact - but this only somewhat helped things)
for Zortch 2 levels I don't use the BSP anymore and I just split away or remake rooms now
after you make a lot of levels it just gets in the way 🤔