Keep watching the floor plates, in which sequence they raise and disappear. You just have to wait until the two floor plates down to and next to the further away gate are raised, then just quickly run down them and right up to the gate. After that you're safe from falling, you can open that gate and when you're in, open the other one with a lever next to it to save the troubles for the future.
It's quite easy once you've figured out what and how to do.
To avoid pressing CTRL+F12 each time you play the game, set the amount of cycles in dosbox.conf file to 20000-25000, that should be perfect for the game. If you have already more cycles than that, you don't need to change anything.
As for which spells you can cast with the stones you have, it doesn't seem to be available at Abandonia in the extras for the game, unfortunately.
Just download this complete list of spells for Ultima Underworld II (edited by me) from here -
Link. It has all spells standing there, hidden and known alike.