The fatal flaw of Merrily is the basic conceit of telling the story backwards.
None of the characters are particularly likable at the "end" of the story, that is, the beginning of the show. It is into the second act before the principals become sympathetic.
This was one of the many unsolvable problems with the original production and is the deal-breaker of the revised version that is the official Merrily being produced nowadays.
As for the 'young' cast, the original felt like it was a bunch of kids playing dress-up. You've got to have adults play this show.
For the record, I saw the closing night of the original and have seen the revisal twice--at The Shaw Fest and Kennedy Center. No matter how good the cast and direction, there is that book.
The score is the thing: singable, emotional, not cerebral, and at times heartbreaking. I hope Lapine lets it shine and may John Doyle keep his hands off our musicals forever.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher