You only have to unlock it once. Then it's forever unlocked as an option on all new game files you make.
An important warning: Carrying over data will remove all your current game's stored items (in every place). Everything is removed and then overwritten. The New Game+ file will probably steal stuff from the old file. You may want to make two copies of the old file and delete one once you have all the stuff from it.
SUMMARY: When you import from another game data file, you carry over all your stats, skills, items, money, and shipping history. The more important aspects of your field's status are also preserved as well as villager's statuses and equips. The things that do not carry-over are mostly scenario-related: story progress, villager LP/FP, revealed areas in maps, and all of the orders. One curiosity is that recipes are not carried-over, which means you have to be prepared to stock both regular and high-level recipe bread to be able to start off where you left.
All events trigger at random. The marriage event will not be able to be triggered until the player is currently dating a candidate, have gone on a minimum of three dates, have seen the sub-event, and have their LP at 10 or more. LP will increase much slower from 7 points up if you are not dating a person. When you have multiple boyfriends / girlfriends, it's best to try to confess at 10 LP.
You can give 1 neutral, 1 liked, and 1 loved gift to each person everyday.