You can preview any theme installed on your site with the built-in preview feature. There’s no need for a plugin. A plugin though might offer more features than the built-in preview.
Since you have a local installation, changes will not impact other users, so you could simply activate any installed theme. Experiment with it a little, then switch back to your original theme. No need to reinstall WP just to switch themes.
The WP Staging plugin basically installs a second WP installation for you, so in essence it violates your no installation criteria. It’s not that difficult to manually install a second instance of WP in a subdirectory of your local server, but the plugin makes it even easier.
While a multi-site WP will let you have multiple sites with one installation, I advise not going this route unless you really need a production multi-site anyway. The reason being is it’s difficult to convert a multi-site back to single site. Multi-site involves some quirks which are best avoided unless you really need multi-site anyway.
The Health Check & Troubleshooting plugin, in its Troubleshooting mode, will let you switch themes and activate/deactivate plugins for your view only without affecting what other users see. Not much of an advantage on a local installation with no other users. They way it works is interesting though, it basically sets up a temporary multi-site installation. When exiting Troubleshooting mode, the multi-site configuration is destroyed. It somehow gets around the difficulty in manually reverting back to single site. But it’s not a true multi-site, we cannot add our own sites to it. You do get a second site to experiment with without affecting your original site.
