I think there is a way to kind of meet halfway in the middle, and still maintain the intent and integrity of the narrative of Part II’s story.
Maybe intersperse all of the Abby flashbacks post her father’s death throughout the season, showing relatively happier times, ala Lost. Still have flashbacks intro each episode, just like what they’re doing with this season, and have the present day stuff split the focus between Abby and Ellie. Hold off on that flashback until the very end of the last episode of the season, just to keep people in suspense about what she’s doing in the present. Then, when you get to the finale, after Joel meets his end, immediately cut to the flashback of Abby finding her father dead, thus giving you context for why she is on her revenge quest. Then, cut back to Abby in present day, after having done the deed, leaving with her group. Last scene is of Ellie weeping over a bloodied, beaten, dead Joel. Somber music. Cut to credits. End season.
You still get quite a bit of info on Abby, her relationships, her personality, etc., but still keep the big reveal until the very end of the season. So, you maintain the gravity and dramatic impact, but a heck of a lot more context. That way, you aren’t left completely wondering who the hell this person is and why you should care about her, but her motives aren’t completely made clear until the finale.