So, as far as your question about finding closer Bonemass spawns goes:

A Vegvisir will only reveal the closest spawn to it. If another Bonemass is revealed, it's going to be just about as far away.

SOURCE: I have two elder spawns revealed - one to the north of my initial base, and one to the southeast.

Also, serpents should not be a problem if you can build a longboat. the iron quality bow does a pretty good job of chunking it down, even with lower quality arrows.