I like that you added a section with historical facts. Although I have yet to find out when the mountain was named or who named it, I did find out the following information from local public library archives:
The word Pilchuck (sometimes written as Philchuck) comes from Chinook Jargon. The word means "red water." Several waterways (at least two creeks and one river) within Washington have either the Pilchuck or Philchuck name, in reference to dark water caused by vegetation or minerals.
I also found this...
There used to be two small communities named "Pilchuck" in northwestern Snohomish County. One community of Pilchuck was located north of Monroe along Pilchuck River. Another community of Pilchuck was located northwest of Arlington along Pilchuck Creek.
SOURCE: (Margaret V. Reid, "Snohomish County Localities", 2004)