Merging 17 Libraries for iCloud Photos access

The iCloud Photo Library is defined to work with the Photos System Library. Only one library can be the system library, so if you want them all on the iCloud Photo Library then yes, medging your libraries is the ay to go.

If you're on a MBP M4 then you should have plenty of heft to manage a 100k library, but as libraries grow there can be performance issues. There is no reason to assume that you will have a corrupted library with 100k images any more than you might with 10k. However, the protection against a corrupted library is an up-to-date back up. (And here I insert my standard warning: The iCloud Photo Library is not a back up, it's a sharing service. Be sure to always have at least one up-to-date back up on another disk, locally)

There are other apps that can do what you want, and are also more powerful. Adobe Lightroom CC or Mylio are both powerful apps but they involve cost. FWIW I have 105k images on Lightroom CC and it works flawlessly.

If you import Libraries into Photos using Photos>File>Import, then you lose all the album structure. But if you use PowerPhotos (as most of us with multiple Libraries do,) then the album and folder structure is preserved, as well as keywords, captions, and titles. Face ID doesn't ever transfer between devices-- it depends on a re-scan. It'll take a long time. Same with object categories and the like.

Are you really sure you want all your pictures together? I have lots of pictures that I mostly don't want to see, because they're bad, or there are just better ones of the same stuff. Like a number of others,I only share my favorite pictures with iCloud and my other devices-- I have a single Favorites Library. But still have many Library in case I to find the others some time.

1. PowerPhotos can definitely do the job. Of course one person's User Friendly is another's overly complex, so that's one only you can answer for yourself. But I will say this: I've never seen anyone complain about using that app
2. Absolutely, go in increments and only after being sure you have back ups.
3. To simply quote the PowerPhotos web page "Got a whole bunch of libraries you want to consolidate into one? PowerPhotos lets you merge your libraries together while weeding out duplicate photos in the process and keeping albums and photo edits/metadata intact. You will be shown a preview of what your merged library will look like before any modifications are done, to ensure your merged library looks how you want."
4. Sorry I don't use the iCloud Library except for testing stuff here, as I mentioned above I use Lightroom CC. But I do believe that albums are synced to iCloud too.

Like you, I keep "Download Originals" on my Mac so that I can back them up. "Optimize" may keep only smaller sized pictures, so a backup of that is pretty worthless. I also keep Originals on my iPad, because it has the room. If I needed the room, I'd use "Optimize." But I don't do much with Photos on my iPhone other than looking at the pretty pictures, so I don't buy gobs of space on a phone, and I use "Optimize" on the phone.

By far, iCloud is way superior to using a cable to transfer pictures. I don't think about transferring -- it just happens. I do have to remember to transfer pictures to my archive Libraries, and delete the silly ones from my favorites Library, but that's pretty minor.

I don't use a shared library, so I'm no good for that. It would be worthwhile to post that as a separate question so you get answers from people with real experience. If you get an answer from léonie, the real expert here, believe what she says!