So I made a map for myself to pinpoint the remaining 21 I have left to complete the Top 100 x 300P and Top 100 x 400P. In so doing, I discovered just how notable the NHN vs. NHG situation is and one need look no further than 1051-ft Golgotha Butte. Its NHN according to LoJ is Horse BM 20+ miles to the north. But the NHG is only about a mile away to the west at the east end of the long E-W crest of Alder Ridge. Alder Ridge is on the other side of the deep canyon of Alder Creek, hence why Alder Ridge is not Golgotha's line parent.
My state map shows that, with the exception of Pt. 400+ near Gig Harbor, all of my seven remaining 300-399P isolation peaks to do are in south-central Washington. Most of the >399P ones are also out that way. With a little perserverence and time, I could probably knock out the 21 remaining peaks in three or four multi-day trips (well, except for Gable Mountain). Some look fun, some are mild trespasses, and some look boring.
BTW, I "got" Lake Serene Hill last Thursday...the summit is occupied by an automotive garage...and is not where indicated by the map at the motel next door. "Thankfully" the summit is not in the fenced-off enclosure where the big tanks are.