Robin's Elwind customs are Gliding Elwind and Soaring Elwind. Soaring Elwind sends Robin sky high on the first Wind gust, but he takes so long to throw the second one out that he's set for punishment. Gliding Elwind /could/ have some use against Rosalina, as the sudden shift in horizontal direction can mess her Bair and Dair and Luma attempts up, but the vertical lift GE has is not all that great, and Robin's better off making use of his slightly above-average air speed to maneuver his way back while still having the chance to throw moves out.
The Nosferatu customs, Distant Nosferatu and Goetia, are impractical. Goetia takes much too long to start to make the windbox worth it, and Distant Nosferatu doesn't have the healing scaling Nosferatu does. Robin wants regular Nosferatu for the scaling, and the chance to throw out a "reliable" command grab without Luma interfering.
Thunder+, or specifically, Thoron+, can (barely) be GP'd on reaction, and if Robin's close enough to where Thoron hits him, Robin just saw his stock hit the road at 20%. While the chance to kill Rosalina at 40% is alluring with Thoron+, he sacrifices consistent projectile flow that he gets with regular Thunder and Speed Thunder.
Arcfire+ is bad. Fire Wall ignites a hitbox in front of Robin's feet immediately after casting it, giving him a panic option, so to speak, and there are situations where it could be used with Rosalina. Depends on the player, really.
Robin's approach is not bound to just his projectiles. His lagless Nair and Fair give him enough reach to complement his Thunder spells and Arcfire lobbing to where he can get somewhere along midrange, while also having a fast backroll to bail out if things get dicey. Rosalina can GP and avoid the spells easily, that is true, but Robin's almost immediately back in neutral after that commitment so he can try again. If Rosalina wants to get optimal punishes, she has to act during the startups of the moves, and not when the moves are already out.