In my opinion? No. Sure, they become a couple and start doing couple things, but it seems as if their relationship stayed the same through all the ~150 chapters they were together. It's kinda like the author made them a couple just for the sake of making them a couple, and them being in a relationship changed them in no way whatsoever. And to answer your third question, the blonde is still the most important person in MC's life; he still kisses the other girls as that's kinda needed in this manga, but his girlfriend always understands him. She's jealous sure, but she's always ok with it since she trusts him.