First off, I don't agree with being able to kick people in the middle of the PQ for various reasons, not least of which is tha tthis would be abused to solo the PQ (thus defeating the purpose). I also believe that there are far more nefarious means to punish people who will go into a PQ and AFK (or any other training map for that matter).
The way this would work is that as long as you are an active party member, you will continue to gain party exp. Once you go inactive you stop gaining exp from party kills, pretty straightforward really.
An inactive player will be defined as a player who:
* Has not killed a single mob in the past 5 min (for regular maps) or 1 min (for PQ and boss maps) with an active attack (summons, DoT, passive damage, souls, familiars are not active attacks. Kishin would not count as an active attack, just to make that clear for kanna leechers). Dealing any amount of damage to a boss counts as killing a mob for this definition.
To discourage simply having inactive buff mules in your parties, an inactive party member will still count as a normal party member when calculating the exp reduction you get for kills. This means that having buff mules in your party at SDH will still decrease the exp you get from leeching even though they won't be gaining any exp themselves. This is primarily to combat people who abuse two or more computers to train in a party with their own buff mules.
One of the biggest arguments I see against this is that people can be active in a party by talking to party members and not actually killing anything. I agree with you, this should count as an active party member. However, I do not know how this could be implemented. It would be trivial enough to write a script that makes your afk leecher simply spout off groups of words that look normal enough so that it is again considered "active" and gains exp for leeching. I am trying to avoid leeching, so if someone can propose a solution to this problem then I will gladly consider it.
Bishops do have low damage, but I am of the firm belief, for any game, that if you cannot even contribute to the killing then you have no right to be in that training map. DH is not the only place to train in the game, if your character is not strong enough to train there then I don't think you should be given any allowances to incentivize what amounts to sitting in a chair afk. It really isn't that hard to let them get 1 kill every 5 min.
Perhaps what you would be happier with is a 10 or 15min rule. I don't think it should go any higher than 15min (I really think 10min should be the maximum, but its whatever) though.
To your last paragraph. Just no. You don't even need a script to get around that. You simply turn on netflix and you are using your keyboard and mouse and still getting exp in Maple despite clearly not playing the game???? Absolutely not. I am trying to avoid behavior such as this, and currently the only way to do that is to ensure that they are killing mobs. Like I said, it is not a perfect system. But I do believe that it is a great improvement to what we have now. If Nexon could develop a function that checked for actual conversations taking place instead of random keyboard inputs to the chat box, then I would be ok with calling those members active. But that is a very difficult programming task, and one that I don't think Nexon's developers need to waste their time on quite frankly.