It certainly seems to be possible that Merry is confidently overstating the exact fullness of the Conspirators' knowledge. He doesn't actually say
what Gandalf told Frodo, only that they know it (leaving open the distinct possibility that they don't actually
know) and he sort of contradicts it anyway, right away, when he says, "We know
a great deal about the Ring."
"A great deal" sounds to me not unlike a euphemism for "something, but not everything"--i.e. Merry is puffing up their knowledge just a touch. After all, he *is* clever enough to put together the pieces, and
Frodo certainly hasn't been as tight-lipped as Sam (he needn't just have been whispering about "shall I ever see that valley again" since April)--and a little bit of overconfidence is not out of character for Book I Meriadoc: the same Merry who thinks cutting through the Old Forest is an excellent short cut and who thinks a walk in the dark in a strange town full of Men is a good idea less than 24 hours after being rescued from the Barrow-Wight.
That's the "we can make it work" approach.
In a post a few months ago in the CbC thread for this chapter, I think I had the same suspicion you've just voiced.