Continual problems with Preview not staying default application

If anyone can help fix this for me, you'll be my favorite person ever. Glad to provide log files or whatever else if you instruct me how.

My problem started a while ago, when I was still on OSX El Capitan - I'm on High Sierra now. I can't pinpoint exactly when it started, but it's definitely been months.

The problem is that Preview refuses to remain the default application for PDFs, JPGs, PNGs, basically any file that I've set Preview to open, it won't stay the default. Literally randomly, it will revert back to some other program. Then I will set it back to Preview (Right click -> Get Info -> Open with -> Other -> Click Preview -> Add -> Change All), and for a little while, it will stay opening in Preview -- but literally always, it will revert back to defaulting to some other program. Right now, PDFs keep going to default open in ColorSync Utility and image files (PNG, JPG etc.) keep defaulting to open in Grab.

I have not been having this problem with any other default file association program. None other than Preview. If I set VLC as the default player for MP4 files, it stays that way until I change it.

When this started, I had Adobe Acrobat on my system, which I uninstalled in the hopes that this problem would resolve, but it did nothing -- now, PDFs just default to ColorSync Utility, a program I've never used. But yes, it used to keep making PDFs default to Adobe programs, and now it's ColorSync Utility. And I work with PDFs a LOT, so this has basically made me crazy.

Image files (JPGs, GIFs, PNGs, etc.) keep defaulting to Grab. I go and manually "Change All" to Preview again and again, but it's the same as with PDFs. Preview will simply not stay the default application for these files.

No, I do not notice any rhyme or reason to the timing of when the default file association changes. It's not when I reboot, the laptop sleeps, or anything in particular. The timing is not regular. Sometimes it'll be under an hour; sometimes several hours; sometimes I'll get lucky and get like a day or so of formerly normal behavior.

I had El Capitan when this started, but someone suggested it could have to do with not updating to the latest OS, so I updated to High Sierra. Problem remains. I've even reinstalled the entire OS on support's suggestion. That didn't work either. Problem remains.

Doing this (Rebuild LaunchServices to Fix Duplicates in OS X's 'Open With' Menu) worked, but only temporarily: it did make JPGs and PDFs etc. go back to default Preview, but then the problem will return, over and over again. And when I do this "rebuild LaunchServices" thing, it makes a bunch of the icons/images/whatever you call them (the little 2 photos thing that indicates Preview, etc.) go away until you reboot. I didn't even care about that, if it would have solved my actual problem!

Basically, I've tried everything I can think of and everything support or anyone has been able to tell me so far, so please help if you have any insight!

The Preview version I have, if it matters, is "Version 10.0 (944.4)."

I did notice something possibly (?) odd when I right-clicked on Preview and looked at the info, which is the screenshot I've attached to this post. I see that under "Sharing & Permissions," there are two "Everyones." One has "Custom" privileges and the other just has "Read only," as you can see.

I'm running High Sierra version 10.13.3 (17D102), and am on a Macbook Pro Retina 15", late 2013 (MacBookPro11,3).

Oh, and because "How to write a good question" says to list it:

Processor Name: Intel Core i7

Processor Speed: 2.6 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 16 GB

Untitled:

Available: 8.53 GB (8,526,233,600 bytes)

Capacity: 450.01 GB (450,013,147,136 bytes)

[Yes, the problem persisted even when I had more available storage. This is a 500 GB hard drive, and the other 50 GB is Boot Camp.]

Glad to provide any other info if it might help. Very grateful for any help anyone can provide. Thanks!