I am curious how will Part 3 of FFT/FFXII story handle events of Part 1 and Part 2 if they allow players to jump into it without requiring them to complete Part 1 and Part 2.

My best guess for events of Part 2 since WoL never took part in the battle leading up to how it ended...

Castrum Lacus still fell but it was due to the efforts of the Blades but they still got corrupted.

Bajsaljen, Marsak, and Mikoto complete the events of Delubrum Reginae.

Dalriada is dealt with by Bajsaljen, Marsak, Mikoto and the rest of the resistance. Even Oboro takes part in it as he most likely lead a team to deal with the upper path of Delubrum while the others dealt with the lower path.