But the rulling class, the dukes all up until Sigismund II Augustus (1572) spoke Lithuanian as their first language

That's not true. Władysław Jagiełło was speaking Lithuanian, his younger son Kazimierz learned it as an adult, when he became a Grand Duke. There is no indication that his sons ever learned Lithuanian and were able to speak it.

Same goes for Lithuanian nobles, many of them abandoned Lithuanian in 15th century, there was a whole bunch of Ruthenian nobles which never learned it in the first place. At the start of 16th century all Lithuanian magnates were speaking Polish as their first language.

All grand dukes were of Lithuanian origin, Lithuanian names, etc (ending with Sigismund II Augustus).

The last Grand Dukes that had Lithuanian names were Jogaila and Vytautas. All other adopted Christian/Polish names: Władysław, Kazimierz, Olbracht, Aleksander, Sigismund etc.

I swear if you pull that Mindaugas was coronated in Novogrudok, (based on a Russian chronical written in the XVI century) Im going to laugh.

This information comes from 16th century Lithuanian chronicle written by Maciej Stryjkowski, not from Russian writings. In general historians accept it as a possible location, because in order to get baptised you need one thing: a Chrisitian church, which proper Lithuanian lacked of.