True, half of 90 is 45 while half of 60 is 30. The LCD Deck benefits from the 70Hz display overclock tweak noticeably so going up to 90 is huge. I hope the OLED can at least operate at any fixed refresh between 30 and 90.
Definitely looks to be that way. In this video at the linked timestamp you can see the refresh rate being scaled automatically as the FPS cap is scaled. Looks like the Deck is automatically setting the refresh rate to a multiple of the framerate cap so that frame times and refresh times are kept in sync. E.g. with a 30 FPS cap it sets refresh to 90hz (triple), with a 40fps cap it sets refresh rate to 80hz (double), etc.