Any question answered...

DutchCheese (font of all Gazelle knowledge) says all Trophy frames take 26.4mm seatpost.
My Cross Trophy seattube appears to measure 26.0 using calipers. If, as I assume, this has happened through previously using an undersized seatpost, will I be able to replace the seatpost with a 26.4 without damaging the frame?

Just use a 26.0. If that's what's been in there already and thats what you measure it as, worst case, if it slips, shim it. DC is knowledge is pretty encyclopedic but he could be wrong/this could be different etc. You're the one with the calipers, you decide.