To answer your question on Mosquito Spring, nope, don’t bother. Unreliable in drought years like this one, old use trail now obliterated, it got trashed by an illegal weed grow a couple years back. See post/pic in the thread from one year ago here: https://sgwa.org/forums/topic/grinnell-east-ridge/
Grinnell N and W slopes were totally nuked by the 2015 Lake Fire. Besides the depressing visuals, there’s so much charred deadfall and shrubbery now to deal with along what was the Lost Creek route that–as it just terminates into the nicely cleaned-up South Fork trail which you’ll be taking to the Dollar junction–why not just enjoy a relaxing cruise on the latter from its trailhead? If you do go for Lost Creek –be sure to bring the HD gloves and gaitors!