I can't help with the tart questions either, but I may not understand your description correctly. Are the candied Meyer slices just a topping, or is the entire filling supposed to be those candied slices? I also can't imagine why they'd have gone bitter, unless some enzyme did that during the slow cook. (That can happen with navel orange juice, but I hadn't read about it with Meyers.)
At any rate - if you have enough Meyers and time left to start over, here's a Meyer lemon tart recipe I can recommend: Meyer Lemon Tart, originally published in the Cooking Club of America's magazine, Cooking Pleasures. Meyer lemon custard goes into a tart shell, and IMO it's the essence of Meyer goodness.
