Just my opinions, as always: others will disagree with them.
I wouldn't even think about being in Vegas versus Zion: Zion every time. People who prefer a casino to Zion Canyon are insane. If you have an "extra" day in the Zion area, go to Bryce Canyon as a day trip. Zion is the 3rd most popular National Park in the US with 4.6 million visitors in 2023 (just beaten for 2nd by the Grand Canyon with 4.7 million) for a reason. It is hot in July, however (43C when I was last there) and crowded (unlike Death Valley).
For staying near Zion, you have three options:
a) inside Zion at
https://www.zionlodge.com/ At this point, you'd be very lucky to get anything there. However, the Zion Lodge means you could drive to places in the Canyon and aren't forced to take the shuttle bus.
b) in Springdale. This has the advantage that you don't have to fight for parking because the Springdale shuttle bus will stop near your hotel and take you to the park entrance to change shuttles.
c) outside Springdale eg. Hurricane. In this case, you have to drive to the park and compete for parking either inside the park or in Springdale. Springdale has parking but it can be full and is expensive.
I would not consider something other than a hotel with good reviews in this part of the US in July: if the a/c in your room doesn't work well, you will be miserable.
The hoover dam is impressive but you can drive very close to it on the way from Vegas to the south rim of the Grand Canyon. The hoover dam no longer offers the "hard hat" tours deep inside the dam that they used to but the tours are still worth doing.
The route from Vegas to Mammoth Lakes not going via Death Valley is 95 to 266 to 168 to 395. You have the option of a side trip from 168 to see Schulman Grove with the oldest trees in the world:
https://www.fs.usda.gov/recarea/inyo...a/?recid=21134
US-95 is not an interstate: it isn't "I95". (There is an "I95" but it's not in Nevada, it runs from Florida to Maine.) If you think you will be driving on something that reminds you of I-15, you will find you are mistaken. US-95 is very hot in July and very remote, just not as hot as Death Valley. It isn't quite as remote as Death Valley either but it's still remote. How remote? Well Beatty is a "major" place on US-95 and while Beatty has several hotels, it is not large enough for a proper supermarket. Any drive from Vegas to Yosemite in July will be very hot and often on very remote roads. I'm unconvinced that breaking down because your car hit a pothole on US-95 is a big improvement on breaking down because your car hit a pothole on 190 in Death Valley but it's been many, many years since I've been to this area in July. If it was me given that choice, I'd probably go through Death Valley but drive very carefully and have plenty of spare and full water bottles in the car.
For Yosemite, the reason to be in Oakhurst rather than Mariposa would to visit the the (confusingly named as it's not near Mariposa the town) Mariposa Grove of Giant Sequoias. These are the biggest single stem trees in the world and are huge, beautiful (IMO) and very old. You can take a "tram" tour if you don't want to hike. Other than that, there's nothing world class between Oakhurst and Yosemite Valley and Oakhurst receives huge numbers of visitors because it's the shortest way to get from Fresno/LA to Yosemite Valley. If you aren't interested in the Giant Sequoias, I'd stay in Mariposa. Mariposa doesn't get the traffic Oakhurst does and the drive to the park entrance along the Merced River is more scenic IMO.
One can check the website for cancellations for lodging inside Yosemite and I've done this for Yellowstone and Glacier National Parks. But lots of other people are doing the same thing and rooms that are cancelled are reserved by other people very quickly. Unless you check the website very frequently (I vaguely recall it's updated every 15-20 minutes) you might not get a cancellation and, of course, there is no guarantee.