We were there over a year ago and the sign posted said campground guests only, so technically no. However there are no gates or anyone monitoring so use at your discretion, we didn't see the sign until we were leaving but it may have been moved since.

Yes, it's right before the entrance to the campground. Dump station and two potable water fills, with threaded connection for a hose. Have fun! :)

Yes! We used it on 05/02/22 and did not stay at Mather’s Campground or anywhere in the park.

Yes. I was camping on Forest Service land outside the park, not at the campground in the park, when I used it a couple of times. I did have a National Parks pass at the time, but I do not remember having to show it to enter the park on that side to get in to the dump station. But that was two years ago, so there could be a gate attendant now. But anyone can use the dump station (I'm just not sure if you will have to pay to enter the park just to use it).