I don't think the main thing is to explain the laughter. That'll just make it stop altogether. The important thing is to explain how much you love Indigo, and it sounds like this needs to be accompanied by you paying some rapt attention to him.
Reading your post, it sounds like Indigo is really trying to give you everything you need to be happy and may feel that his strenuous efforts to show you how much he cares are being rewarded by you giving that good energy back to Mr. A and not Indigo.
Of course the way you and Indigo are navigating the situation is a more somber and studied process than between you and Mr.A, because you have more history and maybe more tendency to predict each others behavior.
Laughter is unpredictable. It is harder to fake a laugh than an orgasm, in my opinion. Your relationship with Mr. A is full of unpredictability at this point, and it's exciting and gets your gut churning and your giggles going.
Do something unpredictably wonderful for your Indigo, something that's new and exciting. Ply him with food and wine. I've been with my Catfish for 6 years and am still finding ways to surprise him , and vice versa. The laughter will come when you're not worrying about it, when you're in the moment.
Whenever I see discussions on here about NRE, I feel like the term is a little destructive. I feel like over the years, people and the world change at such a high rate of speed that I have to constantly redefine my relationships, and I am always aware that each day is a decision for both Catfish and I to stay and do this. Not taking that for granted helps me know how to tickle my man's fancy. Sometimes I cheat by actually tickling him. Actually, a lot.
Good luck, I wish you laughter with both of your sweeties.