How to pronounce "论"
It's the 4th in 一概而论, both regions: 

Check. e.g.: 


Mainland: 在线新华字典 
Taiwan: 教育部重编国语字典


(Both URLs point to the '一概而论' entry)

论 does have a 2nd tone reading, but 论语 seems to be the only where it is used.