1. No, she is not. Sakura is a main character, as are the rest of Team 7. Her development has not stopped after the Sasori fight, it has progressed. Unless you've completed stopped reading the manga after the Sasori fight, then I can see why you have come to such a conclusion. I really hope you're not one of those people who think; "No fighting? No development."
Also, I would like to point out that the Konoha 11 themselves are secondary characters; they support the main character. You're right about her not receiving the same panel time as Naruto and Sasuke, but that is a different situation. Naruto is the main protagonist of the series and Sasuke follows in second in line of importance.
2. I too do doubt that she even has the slug sage mode, if there even is one for a fact. Although, isn't it possible for Sakura to learn senjutsu? I haven't studied this enough to know for sure, but to learn how to become a sage, don't you have to travel to a the location of the teacher (Ex. Mount Myoboku, Ryuchi Cave, or in Sakura's case, Shikkotsurin) to learn from them? I'm sure that the current Sage's have learned how to become a sage at these locations. Not only that, but it has been confirmed that Sakura can summon the slugs. The entire "Slug sage mode" concept is a bit of a stretch to me, honestly.