>>>How did Yakov become Yankev
That one is easy. In many countries the letter AYIN was pronounced as an ANG sound . The word Shemah would be pronounced Shmang but with very little emphasis on the G. Therefore the name Yaakov would always have been pronounced as Yankov by them. (Actually Yankgev but with so little emphasis on hte g that it sounded like Yankev)
Even today in Amsterdam children are still taught that an AIYN in ang, Briskers repeat the words in Shema with an AIYN to also make that sound in case it is correct and some of the old time Yekke shuls still insist that whoever davens for the amud pronounces “Elokei Yangkov”.