So, I'm currently attempting to have a progressive conversation with an NPC where the character can choose an option, get an answer and then the conversation topic [shown from a list via the 'Split ("Questionhere;Question2;Question3")' command] and I can't quite figure out how to make topics disappear after they've been chosen by the player? I've tried using the 'First Time' function but that alters the conversation after picking any topic rather than specific ones, which isn't what I want.

Additionally, if any results could be kept simple that'd be grand; I'm still totally knew to all this, so I've no idea what to do when folk start dropping big blocks of code in front of me.

If using the desktop, I suggest using ConvLib, which was created to do just that.
https://github.com/ThePix/quest/wiki/Library:-Conversations

I currently have ConvLib but I'm a bit stuck, truth be told. With the ShowTopic & HideTopic function, where/how do I use this? I'm not familiar with a lot of the niches with Quest so far, so my apologies if this is a silly question.

Also, is it possible to get hidden topics to show up again after the player leaves the room (via room script or some such)? I see that the hidden tag takes priority even if you set it to visible, but I'm just wondering if I'm missing something there.

It is not built in; you would have to set two attributes yourself.

topic.display = true
topic.hidden = false