I just rewatched the end of the Heart of Darkness episode(scene with Emma and Mr. Gold) and then the beginning of the Hat Trick episode(scene where Emma/Mr. Gold find that MM escaped).
Seems to me that Mr. Gold made the deal with Regina to implant the key in MM’s cell BEFORE Emma came to him for help at the end of episode 16. When he made the deal with Regina it was probably to suit his own purposes. BUT, After Emma’s visit with Mr. Gold on that same night they both head on to the Sheriff’s station…probably due to Mr. Gold’s prompting. Without Mr. Gold, Emma would probably be on her way home to rest for the night.
Now, that Emma has revealed to Mr. Gold that she wants to do “whatever it takes” to take down Regina’s charade, he’is on her side now. I mean, without Mr. Gold’s needing to visit MM that night(all of a sudden?? up to that point he was in the middle of taking inventory!!), Emma would not have found out that MM escaped in time to look for her and bring her back before the arraignment the following morning.