iCloud Photo Library: Must Upload All Photos from All Devices? Backup?

I currently am still sync'ing photo albums from my Mac into my iOS devices. I was thinking about turning on iCloud Photo Library, and it pointed out that doing so turns off sync'ing from my Mac, meaning that all of those photos will go away. OK, that's not unreasonable, since the idea is to have them ... well, in the cloud rather than burning up space on your devices.

But taking it further, it started to get ... more interesting: This appears to mean then that, to have access to the photos that I currently sync into my iOS devices from my Mac (as well as a whole lot more photos), I would need to:

• First, I have to upgrade my Mac to Yosemite, right? (That's a little tricky with respect to FCS7, as it turns out, but apparently can be done.)
• Second, I have to upload my Mac's entire iPhoto library up to the iCloud Photo Library too (all ~85GB of it), right? Or do you have an option to make only a portion of it appear in the iCloud photo library?
• Is there some way to verify for certain that all 12,875,837,641,893 photos (haha) from all devices got correctly uploaded before wasting them from the iOS or MacOS device?
• Uploading 85GB would probably take quite a while, I've seen many many times that the longer any process takes, the more likely something will get hosed up in the works. Am I assuming correctly that this upload-everything-and-delete-it process is very foolproofly automated?
• Am I assuming correctly that, after uploading everything, all of the organization options that we're used to on iPhoto on the Mac (albums, events, etc.) all work just as before, the only difference being that all of the photos themselves are "in the cloud"?

Moving everything into the iCloud is probably feasible. 200GB of iCloud space (which should cover it all, I'd think) costs something like $4/month, which, is not the proverbial "arm and a leg," since it also cuts down space required on my iOS and MacOS devices as well. Also neat to be able to do ol' style slide shows (more or less) on the Apple TV. The biggest advantage for me is taking photos or movies on one device, or editing on one device, and having them show up immediately on as modified on all devices. Then again, if I don't have Internet access at any point (ugh)...

But anyway, I'd probably also want to make an explicit, external-HD backup of that library, if possible. Is there some easy way to do that? For example, does the iCloud Photo Library show up in my iCloud Drive, and therefore I can back it up like any other directory?

Thanks for the info folks!