Wow. That last post provided a lot of good info.
Everything in your post seems spot on with what you'd consider ideal conditions for operation. I have only a few more theories as to what's going on. After that I'm stumped.
1. Are those mishimoto electric fans installed correctly? I don't mean it as an insult, but from what I read on the jegs site it doesn't say if they are pusher/puller fans. Where is your A/C condenser mounted? If it's mounted directly in front of your radiator and has a large surface area, it could be interfering with the radiators cooling efficiency.
Being in Vegas means your A/C is probably running as much as your engine is. Are your A/C components in good condition? (Compressor, Heater core, no refrigerant leaks?)
2. Your radiator hoses seemingly being opposite of what you'd expect and you stating that you are starting to hear what may be belt noises is a big red flag to what I suspect is the root of your problem. I hadn't brought it up because I forgot all about it happening to me.
Before I had to rebuild my engine I had just freshened it up after it had been sitting for almost 8 years of not running and with just tap water in the block. I re-gasketed the engine,new distributor, rebuilt carb,new chrome aluminum hiflow water pump,belts,hoses,etc. Everything short of head gaskets. I got 4,000 miles out of it when it blew a head gasket. (They were 20 years old after all. Lol.) Thought a new head gasket set and about a week of work I would be back on the road.WRONG. Long story short it needed a rebuild. Block checked out good. Bored .040 over, New set of reman 882 heads, comp cam XE250H kit and I was back on the road a year and a half later.
Immediately was having hi temp issues. With the hose temperatures being opposite of what you'd expect just like you stated. But was able to keep the needle right in the middle of the gauge, then after about 100 miles started hearing belt noises, replaced belts, another 100 miles, belts again. This time i spun the water pump pulley and fan without the belt on it and i could hear the pump groaning. I wasnt thrilled at the thought of replacing my chrome aluminum water pump after only 5,000 miles or so. So I slapped on a stock cast iron pump. $30 & lifetime warranty, sure why not?
Just like that, my engine temperature woes were gone! Haven't had a single instance where the needle rises above 1/4 on the gauge. No heater core for assist. Stock fan, no clutch. Idling in traffic and 90 outside not a problem.
That damn chrome aluminum pump was a REVERSE ROTATION WATER PUMP. There was no identifying marks on the box or the body of the pump, I had to pull the impeller cover off and check the impeller blades to confirm it.
Yes, reverse rotation pumps are designed for serpentine belt driven setups. But they have the same flange and bolt spacing as standard rotation v-belt driven pumps.
And just because summit says it's the right pump, doesn't make it so. I would make certain it is the correct one.
Also, bad motor mounts will shorten a water pumps lifespan due to vibration and unbalanced rotation.
So even if it is the correct pump, it could be bad already. A pump doesn't have to be leaking out of its weep hole to indicate it's bad. In fact, most pumps have two weep holes. I dont remember why, but you can have a very slight amount leaking from one of them and it'll get blown away by the fan, or evaporate off the block before you can find it. That was the case with my chrome pump. I guess 5,000 miles rotating in the wrong direction will eventually cause something to fail. LOL.
That's all I have to offer for possible causes. I apologize for the long posts, but as someone who has experienced these things personally, and only about 2 years ago, I'd like to know I wasn't the only one to have it happen to.