John and Mary divorce, but become friends (maybe not right away, but maybe they part on friendly terms from the start, it's up to you). They share friendship and custody of their little girl.
Next time John is getting married, it is to Sherlock... With Mary as his best (wo)man. She's happy for John and wants him to have a great wedding, so she makes sure of it. Lestrade can be Sherlock's best man. And Janine can be Sherlock's bride's (or rather groom's) maid and is amused about it.
A secular wedding is highly preferable.
Bonus: Sherlock is a drama queen, as John has established, and even more so during his own wedding. He's nervous and maybe it shows.
John/Sherlock. Past John/Mary. Wedding.
Next time John is getting married, it is to Sherlock... With Mary as his best (wo)man. She's happy for John and wants him to have a great wedding, so she makes sure of it. Lestrade can be Sherlock's best man. And Janine can be Sherlock's bride's (or rather groom's) maid and is amused about it.
A secular wedding is highly preferable.
Bonus: Sherlock is a drama queen, as John has established, and even more so during his own wedding. He's nervous and maybe it shows.