There is at Lake Mead as well as on I-15 at the truck stop.

No. They have a dump at the VoF campground. Do not know if there are costs if you are not a guest.

This is dispersed camping so not near the entrance to the actual BLM site but there is a dump inside the park itself here: https://www.campendium.com/valley-of-fire-dump-station

No, the dump station inside the park is free after admission fee.

Closest one south of VOF on the I15 is located at the Loves fuel station on exit 64. North of VOF on the I15 there's 2 dump stations listed in Mesquite.