When you first started reading Goblet Of Fire, you accepted that Mad-Eye Moody was Mad-Eye Moody, right? But then you got to the end and found out that he was actually Barty Crouch in disguise. He was passing himself off as someone else. What's stopping someone else from doing the same with Regulus? The catch is that this time, they don't even need polyjuice potion or anything else JKR can think of. Everyone assumes Regulus is dead. What if it were Snape acting in his double-agent role who found out that Voldemort had a horcrux in the cave? Suppose he was put into a situation where he couldn't tell Dumbledore and had to destroy the horcrux. Of course, Voldemort would find something out (not that the horcrux was destoryed), and Snape had to give him someone else or be killed himself. Naturally, he gives Regulus to Voldemort, because Sirius' brother was a nobody. Voldemort may know about the missing horcrux or he may not. Given his state for 13 of the past 16 years, he hasn't been in a position to check. Snape could easily have initialled that note as being from Regulus, or even took Regulus with him.
See, everyone assumes that Regulus is still alive, and are all excited that they've worked it out. But JKR has said in the past that by the end of Book 6 we'll have met everyone who is important to the plot. And we haven't met Regulus yet. Since 'he' destroyed the locket, 'he' is fairly important, no? Everyone is on the lookout for a highly-magical solution to the problem because that's the way Rowling has trained us to think. And now everyone is laughing, assuming Regulus is still alive, and thinking how clever they are. I wouldn't be surprised if she says, "Well, yes, Regulus did find the horcrux ... but it was actually Character X using Regulus' name". She's already said nothing can cure death and Sirius made it pretty clear that Regulus is dead, even if he didn't know the circumstances.
I'm not saying you're wrong, I'm telling you not to jump to conclusions. JKR still has a few tricks up her sleeve and I wouldn't be surprised if she's got one for us when it comes to RAB. Don't forget, she said Regulus as RAB would be "a fine guess". She didn't say it was the right one.
