I"ve been trying to figure out the tones for the character 论. It seems like it is fourth tone in 讨论. However, in something like 一概而论 I'm not entirely sure. Is this a taiwan/Mainland division? Or is it decided on a word by word basis?
It should be the 4th tone.
Actually, every "论" has the same meaning is the 4th tone, such as “讨论”(discuss) “理论”(theory).
“论” in the word “论语” is specially the 2nd tone because it doesn't means like that.
When the 论 is the 2nd tone, it should be in classical Chinese and means:
说得尽(some thing you want to say can be finished)(is always used when there is so much thing you want to say but you can hardly finish them.)
表达，抒发(Express something you want to.)