Can I use the free night certificate as a part of another longer booking? Say I book 5 nights in cash at the Conrad Koh Samui in a non base room. Is it possible to use the certificate towards one of those nights?
Or is this mainly useful to 1 night stays in base rooms?
It's not just "useful" to use for base rooms, it's necessary. The stipulation is "standard award rooms only". So in your case you could not use it for one of your non-base rooms.