Trying out 1P 8 one one of my 1P7 license Macs

Hi VT1P!

Thanks for your interest in trying out the 1Password 8 Early Access. :) If you're currently using a standalone vault the first step would be to sign up for a trial membership and migrate your data to that using 1Password 7:

Migrate your existing 1Password data from standalone vaults to a 1Password account

Once complete you should be able to follow the instructions in the announcement post. To answer some of your questions more directly though:

I see step one is to close 1P7 and Safari, then move 1P7 from my Applications folder to the trash. Should I also delete my current Safari 1P 7.8.7 extension?

With v7 the Safari extension is bundled inside the 1Password 7 for Mac application. Deleting the app will also delete the Safari extension inside of it.

If I put my 1P7 1Password.opvault in a desktop folder on my Mac, then download 1P8 and the new Safari extension, and set up a trial account, will 1P8 have the same Sync preference as 1P7 that lets me Sync to a selected folder? Will 1P8 recognize and access the 1P7 1Password.opvault in my desktop folder?

It will not; no. 1Password 8 doesn't know anything about standalone vaults or syncing with anything other than a 1Password membership.

I hope that helps!

Ben