Mt Whitney Zone Forums General Discussion Walk-in campground availability question

Hello,
A friend of mine are day hiking Whitney Friday August 10 and would like to camp at the Portal Trailhead campground Thursday night. I understand that there are only 10 walk-in campsites. The WP Family Campground is completely reserved.

We could reserve at Onion Valley/Grays Meadow 13 miles away, or we could try the walk-in sites at Horseshoe Meadow 14 miles away (distances approx).

We'd rather camp close enough to not have to drive to the trailhead.

My questions:
How likely are we to get a walk-in campsite at the WP Trailhead if we're not there at the crack of dawn on Thursday to grab one?

Do people commonly just camp anywhere in Inyo NF or sleep in their car? (I don't want to do this if not legal/commonly done.)

Thanks,
G