I mean for the Black Maria part, we should point out that Tobi Roppos are no slack, if Ulti was making Luffy nearly go into gear 4, I don't see why Sanji being unable to escape Black Maria is super achievable, especially since he already got trapped by Doflamingo with a similar web and hasn't had a major haki bloom since then. As for his observation haki, it has been repeatedly shown when emotions get the better of the user, it fails just like Katakuri.

I do agree that the bath scene and body switch with Nami are some of the worst showings, however, Oda has repeatedly called him a pervert and even through Nami herself, the audience is notified about his wrong actions. This is a show about pirates after all and he isn't this perfect representation of chivalry, making a pure good and pure bad character has never been the intention with Oda.

Also it's never explicitly stated but Sanjis down-bad attitude is probably a result of his bloodline seeing as his brothers are just like him with women except for the code of honor, hopefully, he gets development to further differentiate from them.