On my wife's computer, v 12.30, build 8360, files open on single-click (not set to do so), and "Don't replace Explorer" does not work (Win 10, x64, Start, File Explorer: starts Dopus even if "Don't replace Explorer" is check, Save button clicked and computer restarted). The only thing that happens when I check "Don't replace Explorer" is that files do no longer open on single-click.
Make sure none of the Windows compatibility settings are turned on, as they can cause Explorer Replacement to malfunction. (See the FAQ on this if you need more detail.)
Is the single-click issue happening in Opus or on the Windows desktop, or somewhere else? Opus and Windows have separate settings for how each behaves.
Mice buttons are also prone to start double-clicking from a single click, especially as they age (but sometimes after only a few months or even weeks, too). If you see it happen in more than once place, that could be what’s happening.