Well, I originally thought of Regulus at first as well, but as mentioned, he really doesn't seem to be any exceptional wizard. When Sirius described him in OotP, he came across as really weak and easily manipulated, and I can't see the same wizard that Sirius mentioned as such being able to get through all that stuff that Harry and Dumbledore did.. Still, I suppose that Sirius could have been giving Harry a sort of different image of his brother because he was ashamed that he was a Death Eater and all that..
Um, as far as Bellatrix is concerned, I really see her as way too outwardly dedicated to Voldemort to ever try to pull that. Sure, she could be trying to pull a Snape, but I really don't think she is that good of an actress.. she just seems way too loyal to Voldemort to me..
I do have to wonder about the two people thing, because yes I agree it would have taken two people unless these people (or person) did change the defenses afterwards, but again the note was in the first person singular tense .. perhaps (if the defenses remained the same) the other person died from drinking the potion..
Also, if it was Regulus, wouldn't Grimmauld Place be a bit of an obvious place to keep the Horcrux? Maybe something in Grimmauld Place would lead to its hiding spot, but then again, the Order did basically turn the house upside down during the time they were cleaning it..