i have never added an mlf culture, only had spontaneous mlf in wild yeast ciders. so i don't know how long it takes after you pitch the bacteria.
you can use your strategy of adding juice or concentrate until it tastes good, bottling and letting it carb up, and then heat pasteurizing the bottles. this is tried and tested but also dangerous if you get it wrong, too much carbonation pressure before you cook them and they explode right then and there, don't cook them through and they explode a week later. if you have space you can sweeten, bottle until carbed, and store in the fridge, and if you don't mind plastic it's much safer and easier to gauge the pressure.
there is a lot of information here