The Progress of Sherlock Holmes (http://archiveofourown.org/works/173274) by ivyblossom is perfect for your request. It's long, plotty, gorgeous BBC verse storytelling, with a pining Sherlock and a cheating Mary, and, as requested, a happy slashy ending, though it takes time to get there.

AfroGeekGoddess's After Life (http://archiveofourown.org/series/11192) series is a BBC fic during Sherlock's years away and after his return that involves a deceitful and dark Mary. The third story is still being posted (a chapter went up last night) but I believe it will resolve happily.