1) The virus is mutating to a less lethal version
2) The heat of the summer is somehow rendering it less deadly
3) The jump in cases is due to increased testing, so more cases are being detected.
I'm not sure where you got 1) and 2).
The affects of the mutations identified so far are suspected to be inconsequential but still unknown. At worst, they can render the vaccines currently under development less affective.
The heat of summer is not rendering it more deadly. It's that positive test cases are skewing much younger to a healthier population while more testing is being done. It stands to reason if testing is limited to people with symptoms as in the early days vs. testing through contract tracing of people who have no symptoms, positive cases per test are bound to go down. But are they? Not is some places.
Old folks and folks at risk are taking greater precautions while younger, healthier folks were partying like it's 1999 anywhere social gathering prohibitions were lifted. It's sociopathic behavior--the youthful sense of invulnerability is understandable; obliviousness to the risk of passing the virus on to another is not. It's not unlike drunk driving.
This is nothing like seasonal flu with the evidence smacking you upside the head since March.
3) is certainly correct as noted above. However, the severity of the pandemic is best measured in hospitalizations and the death rate will eventually climb as ICU admissions climb, with some lag time. Climbing they are in current hot spots, to critical levels. If ICU bed demand exceeds supply, which it already does in some places, the death rate will be exacerbated.
Also, it is becoming quite clear this is not a disease of impoverished inner cities and nursing homes. It was recently reported in Erie County, NY (Buffalo region), that white suburbanite positive tests are now coming in at a higher that black inner city residents. It's the difference between early awareness (and fear) vs. complacency. Wherever you find complacency you will find spread. Complacency might have been understandable in March. You have to be pretty stoopid at this point not take recommended precautions.
When all is said and done, it can be like modern war casualties--20+ disabled for every 1 killed. You may get sick from it and recover but that does not mean you will not have permanent lung damage with more medical costs and premature deaths down the road.