Mevatel D'rabonnon l'chatchila? (Shabbos 29a)

Rashi (d"h marbe) says that when you add pre-cut wood to the fire with wood that fell on Shabbos it is ok because you are mevatel the muktza wood b'rov. Isn't that a problem of being mevatel isur l'chatchila? The Rashba asks another question: How can it be batel if it's still nikar b'shas tashmisho? The Rashba therefore has a different explanation of the gemara. By adding more wood, you make it doesn't look like you're moving muktza and you're really just being metaltel min hatzad. I think he's saying that tiltul min hatzad alone would not be enough. Even though that alone takes off the isur muktze we're still worried about other people seeing. Therefore the gemara says you need rov so that people won't see the muktze.

The Mossad Harav Kook edition points you to the Rashba in beitza on daf 4b. There, the Rashba says basically like Rashi here except that he says you can only be mevatel it b'rov l'chatchila as long as you can't see the muktza wood. He says that the normal rules of bitul don't apply to d'rabonnons "d'ein lahen ikar b'doraysa klal." The MHK there says that the Rambam implies that the regular rules of bitul don't apply to any d'rabonnons.