How would Sherlock and John do fake dating for a case? How long before feelings are confessed? If provided 2 whole beds, would they come up with excuses to share anyway or only share after confessions? How would things change if Mycroft sees them and fully

It all depends on the specific circumstances! I've actually written two versions of fake dating for a case, one where the circumstances helped them enormously almost from the outset (Bridging the Ravine), and one where they exacerbated the issues they already had before they got themselves sorted out (The Bells of King's College). I've even done one where they started dating for real during a case but had to keep it a secret because of the case - aka, the reverse! (The Four Horsemen of the Apocalypse). There's no one and only way it could go! As all the other fantastic fics written to the same trope would attest! :)