When UPdating Dev Channel PORTABLE version of Chrome..ALSO how about/how will THIS UPdating, affect the Portable STABLE v?

So how will I know, down to the individual extension-level, which extension is portable, and which is not?

You can't. Not unless you personally test out each and every one of them by checking if they are portable.

Should I had even cared to ask this question? Is it when I install the individual ext? Some behavior gives it off?

There's no "behavior that gives it off", while you're installing it. You can't tell if an extension is portable until you test it out with different drive letters (i.e: moving from one drive to another). The simplest way to test is installing an extension when the program is located in (for example):

D:\PortableApps\GoogleChromePortable\

You change some settings of that extension, save them and exit the application. Then you change the drive letter (works also for moving the entire folder structure to another drive), and then it would be something like

E:\PortableApps\GoogleChromePortable\

Now you look at whether the changes you made persist or not. If not, it's not portable. If yes, then continue testing. Now, while in a new location (E:\), change some more settings of that extension, save and exit. Now move the entire folder structure back to D:\ and check whether the settings made both the first and second time persist. If they do, it's likely that the given extension is portable. Likely, but not certain - you would need to test it out a lot thoroughly to determine with certainty.
Note: when I said move, I meant move, not copy to other drive. I could explain the reasons, but don't want to make this longer than needed.

...if any of us find an extension NOT portable, can we ask the author to try hard to make it portable now so we can ''bring it with us too''?

Well, you can always try with a polite email asking the author to try to make the necessary changes to make it portable, but sometimes it just won't help. On the other hand, never underestimate the power of persuasion!

''so will not interfere with the other'': this applies vice versa too? Like when updating the STABLE Portable version...it effects on the DEV Channel VERSION?

Yes, it applies to the other way around. As long as you're not running ANY version of a software you're updating (so close both the "development test" and the "stable" versions of GCP before updating any of them), it should not be possible for one update to mess around with the other app. They are separate apps, in separate folders.

Note: The method of simple testing of the extensions' portability applies to other browsers as well. This is a simple way to test, and can be done on all portable browsers.