So how to replace CMYK (0, 0, 0, 100%) black with RGB (0, 0, 0) black? Is there any tool (including in Creative Cloud) that can accomplish this simple task on all text and objects in a PDF file?
From the tests I've done using Acrobat Pro, you can use the Preflight tool and have your entire document converted to RGB. The K100 were changed to a real RGB black on the tests I've done.
You can explore the option below, maybe you'll find a way to specifically target only certain kind of blacks. The option below will convert everything to RGB.
To do this:
Find the Preflight panel in Acrobat Pro
Click on "options" on the top right of this panel to find "create new profile".
On your new profile, find "fixups" in the list on the left. You will see a list of conversions you can apply on the right side. Add the ones you need to your profile by clicking the light blue arrow on the bottom. The first one to use for you now is something like "convert to srgb". You might need to make a few tests with the kind of fixups you want to add to that profile and see what works for you because there might be other fixups you'll find useful.
You can also change some fixups settings by selecting them in YOUR list and editing the settings on the bottom right.
Then save that profile.
Finally, you can use that new profile by selecting it and then clicking "analyze & fix" on the bottom of the preflight panel. That's when Acrobat will convert your document.
You will be able to use that same profile later if you have other files to convert. I think it's also possible to create a droplet.
Hope it works for you!