Sentence Correction

I have few questions with a question.. please help me clarify..

Q:

The Glass House Mountains in Queensland, Australia, were sighted in 1770 by the English navigator Captain James Cook, by whom they were named supposedly because its sheer wet rocks glistened like glass.

Questions -

by whom they were named supposedly because its sheer wet rocks glistened like glass. - Is this a dependent clause? If yes, what is the best way to think of connecting it to the independent clause?

whom - I am not sure whether it should be whom or who

they and its (I figured out 'its' should be changed to 'their') - looks like both 'they' and 'their' refer to "Glass House Mountains" - is this correct?

any suggestions?

I am not sure if I really understand your question, but the trick I use to determine between who and whom is to substitute them with he/him.

In this case, you wouldn't say that the mountains were named by he, but rather by him. So I would choose whom over who.