What title did the merchants who dealt with Canton's 13 Factories have?
What title were the merchants who had authorization to deal with the europeans in the 13 factories called


they're called 行商 (hong/factory merchant)

indeed, there're several chinese books about the thirteen hongs (十三行); for an english one, have a look of "the 'fan kwae' at canton before treaty days, 1825-1844"

if you can read literary chinese, try the book 夷氛聞記, by 梁廷枏:

https://zh.wikisource.org/wiki/夷氛聞記/卷01


  what's a better name than "the period leading up to the Opium Wars" for this period in Chinese history? 


in westerner's term, i think "before treaty days" is quite good.

in chinese's term, i would suggest:

嘉道年間 (1796-1850)

道光年間 (1821-1850)