Is the only way to get here if the Guanella Pass is open, so you can't hike up the mountain until the pass opens in May?

Yes, I wouldn't go during winter. Regular road maintenance on the pass usually starts up memorial day weekend

You can hike in from the dear creek trail head, but it would be 16 miles one way.

Yeah, it's a 14er so you really don't want to try to fool around on it during the winter. I believe there are several 14ers that are open but this one is not part of it.