I've noticed that a few of the KDE applications require long-presses for certain actions. (Eg. Okular and Kmail) While this may be convenient for mobile or touchscreen users, it is incredibly inconvenient for people who use laptop touchpads. Is there any way to allow such actions to use a right click or hover instead?

I wouldn't go as far as to say that it is "convenient" to use long-press on mobile/touch UIs, after all it does require touching and holding (still) for a couple of seconds to activate. More so it is a necessity due to not having separate right/left buttons. Of course long-pressing with a mouse/trackpad is definitely inconvenient (though not much more inconvenient than in touch UIs, just unexpected and requiring new muscle memory) when you could map that action to the additional mouse buttons the user has available. I'd have expected long-press to generally map to right-click, but apparently not?