Hey Purple Alligator, I am in your area 19997 sw 190th street and I have what is predominately a longan grove.
I can tell you two things.
1) Longans don't need to be sprayed for whitefly or ants, that is a huge waste of money and terrible for the environment. Longans in our area are basically care free as long as your occasionally fertilize and prune. And trust me I know, I have over 150 longan trees.
2) Its not snails, its 100%, my personal guarantee not snails. I accidentally introduced cuban snails onto my property and I have to deal with them constantly, but they do not touch mangos, not even seedlings that are sprouting, and trust me, seedlings are snails favorite snack.
This next advice, is speculation, your curled leaves are possibly the result of fungal problems. Most people will tell you that you need to spray most you mangos with copper or some other anti fungal at least when they are young. Our area is not as mango friendly as you'd expect, the humidity levels and soil aren't ideal.
Sometimes curled leaves are the result of nutritional deficiencies, but I assume you already know to fertilize your mangos periodically with 8-3-9 and a chelate micronutrient blend, on our soil this is essential for good tree growth for most anything and does a world of good for mango in their first years.
Finally, sometimes the leafs just come out curled especially if its very windy and the trees are growing quickly, the new leaves are sensitive to wind.
If you are taking good care of your plants any curled leaves are not life threatening.
But, I can tell you without a doubt in my mind its not snails, and that you are wasting time fumigating your longans.
If you want pm and if I have some time, Ill pop by your place and tell you what I thing and you'd be more than welcome to look around my place.