There is a second Stronghold/Spider Mine Complex which I have not explored, just dug down to make sure it's there..
It's on an island:
1414.157 66.00000 2004.551.
AND...!!! There is an Island with a Stronghold, a Spider Mine, and a Nether Fortress: The Eye of Ender actually takes you offshore of this island, so dig down on the island for 25 blocks: I dug down to find it, but didn't stay to explore.