But Joshua Tree would be a great place to stage a disappearance. It seems to be notorious for search and rescue and visitors getting lost. The hiker might even have google it to come up with JT as a convincing spot for a disappearance.
The 9 am to 12 pm time frame may have been on purpose to make it seem as though a fatality was very likely. He could have gone at 6 am, for gosh sake, but if he had gone at that time and disappeared without a trace, SAR might quite soon have concluded that he disappeared on purpose.
The other obvious fact is that Joshua Tree is far out of a convenient loop from Las Vegas and the 3 other parks he and his wife went to. It seems very odd that they'd pick that one. There are so many other places they could have gone to than Joshua Tree, including Lake Mead, Havasupai, Lake Powell, Capitol Reef, Grand Staircase Escalante, Vermillion Cliffs, Monument Valley, Goosenecks.... The choice of JT is strange unless you were perhaps wanting to go to the place where it would be easiest to get lost and not find-able with likelihood of fatality.