Thank you for these links. I am a retired programmer for the IBM mainframe and Unix / Linux for the most part. So yes, I totally know about Open Source and IDE environments in general, as my first IDE was Borland C++ from the early days to Visual Studio today, used mostly for cross-platform developments.
Speaking of Visual Studio IDE, I was looking at the available extension this morning, I notice there's an Arduino extension. I've no idea whether it works on its own or have to rely from librairies from the failed Arduino IDE install, I hope not! Anyway, I'm mentioning pssible alternatives.
A google search showed that this issue has been raised MANY times. If my tone is harshed, you should hear comments outside of your comment about this very specific issue, and I totally understand why. For something as trivial as creating a folder, even in the \TEMP directory.
My only comment to that, as a (now retired) professional programmer, it is incredibly POOR programming to rely on physical properties of the mass storage they're using. That's why modern OSes including Windows strive to bring a level of virtualization. Sure, RAM DISKs behave differently than real physical disks. And so are Network Drives (which I also have, from my SAN), USB keys and so forth. Just let the OS do its thing... and it will work. You even mention it yourself that previous Arduino IDE worked just fine with RAM DISKs.
It looks like the solution is known, but that means some regression with their codes and that's an absolute NO. So let's say that my eagerness to lend an helping hand is not there.
I am returning visitor to the Arduino, as I programmed one about 10 years ago and now there's this small project which an Arduino will do just fine, so let's do it again. So it was the first install on this brand new PC. It got the \TEMP path just fine but stumbled on creating a directory over there. Wow!
I'm using RAM DISKs (and having \TEMP pointing to it) some 15+ years since Windows 7 and across 2 computers. So MANY softwares got installed using that setup and I'm sad to report that the Arduino IDE in its current form is the FIRST that won't install on computers with RAM DISKs. That's a distinction I wouldn't be proud of and I can't understand why they are so stubborn to NOT incorporate fixes to their main code. It's no one's gain to make users angry.
So that's the situation at my end and thank you for your help! I'll evaluate my options whether going for the older version of their IDE or going the Visual Studio route and adding the Arduino extension to it.