Players who know me are aware of my strategy of raising all eligible bachelors or girls to deepest red heart, experiencing all of their heart events and even marriage and then and only then choosing a spouse and allowing the rival couples to marry.
In many Harvest Moon games, if you have raised the eligible party to deepest red heart level, you then must LOWER his/her heart level in order to be able to trigger the rival heart events. It can be quite a complicated and exhausting undertaking, but it is worth it if you want to experience as many events as possible in the game.
I am happy to report, however, that Island of Happiness is different in this respect. I brought all the bachelors to red heart level, made my choice and married one permanently and THEN, much to my delight, began to trigger the Rival Heart Events without lowering any heart level by one iota. At 10 hearts, I can trigger the Rival Heart Events now that I am married and there is no romantic link between me and the single bachelors. As I never triggered these events BEFORE marriage at this level, it is possible that my marriage may be the key to freeing the rival couples to marry at red heart level.
I had been dreading the long ordeal of feeding bachelors items they hated in order to lower their heart levels. I am delighted and relieved to discover that this is not necessary.
In Island of Happiness as in many other HM games, the second rival heart event cannot be triggered before the second year and the third cannot be experienced before the third year, giving the player an advantage if he/she wishes to marry the eligible girl/bachelor, but unlike games like FoMT/MFoMT, or HM DS/HM Cute DS, you will not even meet all the eligible characters in your first year if you do not progress sufficiently in the game. As it takes time to unlock all the characters, marriage in the first year, except to Elliot or Denny, if your character is a girl, is almost impossible. It definitely is impossible to experience ALL heart events with all eligible characters in the first year. I chose not to marry until I had experienced the Wedding Event with each of the bachelors. Your Blue Feather, incidentally, will be rejected if the eligible character is not at full 10 heart level. Orange heart level does not suffice.
N.B. A reader evidently misunderstood the point of this post and made a comment to the effect that she was able to trigger all Rival Heart Events before her marriage, as if I had stated otherwise. Of course, all Rival Heart Events can be triggered while you are single still, provided the heart level of the Eligible Girl or Bachelor does not EXCEED the maximum level. My point was that I was able to trigger ALL Rival Heart Events AFTER marriage with every Eligible Bachelor still at 10 hearts.