Sounds like you've got it lined out, BUT if you still need an option, you can use http://socaltransport.org/tm_pub_start.php
If you enter your a location in Beverly Hills and enter "2525 Lake Ave, Altadena, CA", you can take a bus to the corner of Lake Ave and Fontenet Way. If you walk (uphill, sorry), one mile (approx. address 3381 Lake Ave if you want to Google it), there is an Angeles National Forest Trailhead. Topo map:
A= Entry Trailhead.
B= Water fountain and where trail drops into canyon (not marked on topo)
C= Actual route of trail into canyon
D= Approx route of trail
E= Echo Mountain, approx 2.7 mi from trailhead. You could camp here if you got off bus late in the day. No water.
F= West branch of Castle Canyon. Water available.
G= Inspiration Point. Picnic shelter here. Technically, the Forest is closed north and east of here according to a very small sign. There were a gazillion people out, roundly ignoring the sign.
H= Mt. Lowe Trail Camp. A decent place to camp. No water in creek immediately by camp. You could go west or north from Mt. Lowe Trail Camp, but you'd be getting into the burn area. Haven't been; not sure what it's like.
K= Idlehour Trail heading east. I went down this trail two weeks ago. Decent condition. You do pass through a burned area, but trail passes through burn area into nice front country area with Douglas Firs.
L= Idlehour Trail Camp. OK place to camp. Not my favorite but nice enough. Water available in creek. There's also water available at the creek crossing shown on the map east of point "K".
M= Junction with old toll road (dirt). Head east here toward pt. "N."
N= Random point on toll road.
O= Junction with Mount Wilson Trail. Descend trail to "P".
P= Hoegee's Camp. Camp here or, depending on your mileage "needs," go north over the hill to Spruce Grove Camp which I didn't mark but should be obvious. Sturdevant (sometimes misspelled Sturtevant) Camp is a Methodist Church Camp. Water available but no camping. The camp is slated to be permanently closed later this month after 120 or so years of operation. If you go to Spruce Grove, you'll need to either retrace your steps to Hoegee's the following day or head down stream (water continuously available in the creek when the trail runs near the creek) and then cut up Winter Creek back to Hoegee's. From Hoegee's, you'll need to head back through points "O", "N", and "M" to point "Q."
Q= Henninger Flats. Two separate camping areas here. Flush toilets (lower camping area only), pit toilets, and running water available. Lower camping area is overly civilized. Even has an electric "streetlight" in the middle of the camp that is on all the time. :( From here continue down toll road toward "R."
R= Random point on toll road.
S= Exit Trailhead. Locked at dark. Might want to check on hours with LA County Fire Dept. who runs Henninger Flats. LA County Sheriff, Altadena Substation, may also know gate hours. There are ways around gate after hours via Altadena Crest Trail.
T= Intersection of Lake Ave and Altadena Drive (a major intersection)
U= Bus stop on corner of Lake Ave and Fontenet Way. It is approx 1.5 miles from exit trailhead back to the bus stop.
I've mentioned places to camp, but obviously I haven't worked out an itinerary. This is just some raw data. Up to you if you want to form an itinerary out of it. Hope it's helpful for this or some other trip.