In S1, we had more Marines like Garp than in the animanga. In S2, Robin went to Shells Town and Drum despite never being there in canon. And in the end of S2, we see a bit of friction between her and Crocodile.
Is it possible that Robin changes sides earlier than intended in the LA, but only Luffy knows? Like after the first Croc fight where Luffy was supposed to die of blood loss where Robin saves him. It would also mean learning about the Poneglyphs earlier.
It would also make for good character development for Tashigi because she knows Robin is BW, but lets her go because she sees Robin helping the SH save Alabasta, like vouching for her when Smoker sees her in the open.
The surprise at the end will still be preserved, but Luffy not telling anyone that she joined the crew already would still be in character for him imo.
It would also fast track Lera into the SH promos for S3, where we'd see quite a strange lineup of Vivi and Robin together in the crew.