Hi Scott,
Sounds like a fun & somewhat lofty goal! Not sure about trails / roads leading up to Idyllwild, so that would make the difference.
But, as you may know, you'd have to walk a long way on pavement, through town and up hill, to reach Humber Park / Devils Slide.
Deer Spgs, begins right next to Hwy 243, and close to a possible camping area. It's my fav trail up there, (huge Cedars), but that, like Marion, (which also starts pretty close to the Hwy and may not require much town walking), commits you to heading up to Wellman's Divide, then to the tram.
It's quite a bit easier to head to tram via Hidden Lake, as you top out around 8,800 ft. instead of 9,700 (Wellmans). Have not been that way since the fire, heard parts have become painfully over grown with some thorny stuff. I was on the Round Valley / Summit trail last summer, and it was in crappy shape from flooding, use, lack of maint. etc.
South Ridge tr. requires a long slug from Hwy 243 to trailhead, but it puts you in close prox. of Saddle Jct & the route to Hidden. Plus, you can check out the Fire Lookout tower while your up there.
If you plan is to ride the tram down?, remember their still on a reduced schedule, (I believe, last tram down at 8pm).
Keep us posted on the route you come up with, sounds interesting. S2