min-height: 0;
might help if the issue were the submenu failing to be hidden when it should be. I don’t see how it would rectify a failure to expand problem.
I suspect the problem lies within the script used to manage mobile menu behavior. When I try to expand a submenu a number of script errors get logged in the console. While the errors do not affect behavior in some browsers, it’s quite possible that certain browsers like Safari are affected. The errors are attributed to superfish.js which is a jQuery menu plugin (not the same as a WP plugin).
Superfish is bundled as part of your theme. While there is a more recent version of Superfish available, it’s unclear how it may have been modified to integrate it with your theme. The chances of you successfully updating just the Superfish component are rather small. There’s no guarantee a successful update would even resolve your problem.
The way mobile submenu display is managed is kind of odd to start with. It’s initially hidden with an element style attribute which normally cannot be overridden. The submenu display apparently can override the style attribute with display: block !important;
. I was not aware that is even possible! Even so it strikes me as an inherently weak approach.
I’m inclined to recommend switching to a more up to date theme that is actively supported. Your reluctance is understandable due to your previous experience. More recent versions of WP should refuse to activate a theme if doing so would crash the site. Even if a crash should occur, since you have FTP access it’s fairly simple to rename the problem theme’s folder so WP will need to use a more suitable theme. You then can regain access to WP admin where you can try activating another theme that won’t crash your site.
You can improve your chances of success if you first deactivated all of your plugins before activating a new theme. Once your preferred theme is active you can then re-activate your plugins. Once again, if activating a plugin should cause a site crash, WP should refuse to activate it. And if a crash should happen anyway, using FTP to rename the problem plugin’s folder will let you regain access to WP admin.