I voted yes, only because if it was Regulus and an unknown, the fact they did what they did shows that not all of the bad guys are afraid enough of Voldemort to stand back and not take action against him. JKR gave enough clues to the fact that there was more then one person there the night they stole the locket. As for the potion being replaced - I'd say it's a magic potion and basin. What could be the most important is not the potion, but the basin itself. Maybe the basin refilled itself after R.A.B drank the potion and removed the locket. After all, whomever knew enough about Voldemort and his Horcruxes also knew he didn't have to possess them in order for them to work.
Once hidden, he would never need to go out and look for them in order to activate them, so R.A.B. would not have filled the basin with a potion meant to kill Voldemort. I believe the basin was filled with the potion at the same time the locket was placed there and it was charmed to refill itself after the person drank part of the potion, lost their mind, and wandered into the water with the Inferi.
We saw the effects of the potion on whomever drank it, so Voldemort, being the solitary person he is would have thought that any "one" person who might try to track down his secret would not last long enough to drink all the potion and take the locket. We saw how Dumbledore lost all sense of who he was and where he was. If he'd been alone, would he even been able to force himself to drink all the potion, take the locket and then leave the cave on his own accord? I doubt it!
R.A.B. had to have other motives for stealing the locket at the risk of his own life. The letter left with the locket gave the writer sole credit for the theft of the locket, but in no way does it indicate that the Horcrux inside was to be destroyed. Maybe the importance in R.A.B. is not so much who they are, but why they did what they did and what role their actions will play in the final book.