I have the first crystal from the first mermaid, but can't seem to find any others. I believe that I have to leave the one I do have in the first slot in order for the path to actually open in the revolving room; so where do I get the next one to put in the latch in that path? Been 3 days a'lookin. Starting to feel like a complete dolt. Help.

Alas - the placement of the glowstones is somewhat random.
If you're lucky - follow the link in my signature and choose hints - there is a list of common locations for the glowstones.
Hope that will help you!

Wyv

Jon,

Been there! Here's an extract from my walk-through...

You will need to use two glowstones at this point (or attempt to block one of the rotating room's doors with a chest). The placement of the second glowstone in Collasium is random and, in a few cases, it is not present. If you have not found a second glowstone at this point by searching all chests along the way then you could reload from the game saved prior to entering the city. However, some players have reported successfully using a chest to block open the east or west doors to the rotating room. Assuming you do have two glowstones then place the second glowstone in the slot near the east rotating room door (this opens the north door). Enter the rotating room, exit east and recover your first glowstone from the slot, and then re-enter the rotating room and exit north.

A "hidden" chest is in the big room past the west mermaid high in the rafters...

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Rick
Hints, Walk-Throughs and More at The WWHAP