Thank you for all the time/attention you gave.
You're welcome.
I downloaded El Capitan 2 days ago.
Okay, but you definitely need to verify that your favorite, irreplaceable apps are compatible with El Capitan (or any OS newer than Snow Leopard). If they aren't, you will have to buy new versions, which I'm sure you don't want to do. [There is more on this below.]
Will clicking its icon and moving through with it fix this (get rid of SL and the Finder quit unexpectedly problem)?
I doubt it will fix it, but I can't say for sure. Installers don't usually replace old or missing "com.apple.finder.plist" files with new ones. In my many years of experience, upgrading a defective OS to a new OS always makes matters worse in the new OS than they were in the old OS. That is why I always do clean installs on my own computers.
If your Finder glitch will allow it, and if you have lots of free (unused) disk space, you could open Snow Leopard's Disk Utility and add a temporary new partition (of 20 or 30 GBs?) to your internal hard drive and install El Capitan on it (if you don't know how to add a partition, let me know). This will leave the Snow Leopard partition untouched. Then you could test El Capitan for a while to see if it will work for you. Any apps that aren't compatible with El Capitan will have X's over their icons. If there are no X's over them, you may still have to copy them (and any associated Library files) to the El Capitan partition before trying to use them in El Capitan. Watch
this short how-to video on doing a clean install of El Capitan. He even does it on a separate partition, if that idea appeals to you.
Before you install El Capitan (if you decide to do it), you might want to consider backing up the "Install OS X El Capitan" file onto a USB stick. This is necessary if you want to keep it, because the installer in the Applications folder will delete itself after it is finished doing the installation. By putting it on a USB stick you won't have to download it again if you ever need it again. After the upgrade, you can turn the USB stick into an actual bootable installer disk (it's too wordy to explain in detail here why I think it is easier wait until after the upgrade to do this).
Will I get an option to either upgrade or clean install? is so; I wish to clean install.
By default, you will initially get only the upgrade option. In order to change it to a "clean install," you will have to startup from the installer and then,
before doing the actual installation, open the Disk Utility app that is
on the installer and use it to erase the hard drive (
I repeat, do this only after you've cloned/backed up your internal hard drive to an external hard drive!!). Then you would quit Disk Utility and continue with the installation. If you create a second partition on the internal hard drive, then you won't have to erase anything. Just select the new, empty partition and install El Capitan on it.