I won't disagree that it feels more disjointed, especially as they bring together so many different small story lines. Act1 is the worst for this, Act2 slows that down a little.
It's not a different show, but things have changed - Jayce says as much. It's no longer Piltover vs Zaun, it's bigger than that now, so the story is about the bigger threat. I mean, I don't think I'd be too bothered about that disagreement with my neighbour if WW3 kicks off, you know
Regarding the other points
The Mel stuff gives answers about her Mom's arrival and cements Mel's character (don't want to say too much).... and no doubt gives some context around where the show moves next
The Jinx change is because after losing both her adoptive dads and her sister (to Cait), she's finally got something to care about again. It's all part of her redemption arc and why her relationship with Vi changes...it's discussed a bit more later in the series
Silco was brilliant, but I think having another Silco-type main 'baddie' would feel too similar.
End of Act2 might change your mind a bit, I found Act1 to be the weaker part of the overal season.
I think it would have all been OK if s2 was 2 seasons.
Time to bring the characters in, to build a relationship and emotional bond with the viewer.
My problem was I had strong feelings for all the characters in S1. But in s2 even the characters I felt feelings for ebbed away. Probably caitlin the most.
The changes in jinx were too quick. And this goes for all the main characters imo. And same for the new characters. They were thrust in and then gone.
I liked the slow pace of S1, obviously everyone loved it. It was so popular. Just wish s2 could have been 2 seasons
