Halve the jalapeño peppers and seed them for a milder flavor or leave the seeds and ribs intact for extra-spicy. Slice the peppers into thin strips, then finely chop the peppers.
Chop or thinly slice the scallions and separate the whites and the greens.
Heat a large nonstick deep or wide skillet over medium-high heat. Add oil, 2 turns of the pan, melt butter into oil, and when it foams, add the squash, zucchini, jalapeño, corn, and scallion whites and cook to soften until squash is tender-crisp, 5 to 6 minutes. Grate the peeled garlic with a handheld micro-grater or crush, then finely chop. Add garlic and scallion greens to squash and corn.
Preheat broiler with rack at center or turn oven to 500˚F.
For the Mexican cheese sauce, in a medium saucepot, melt the cream cheese into milk and stock over medium heat.
Meanwhile, combine spices in ramekin or small dish. Add about 2/3 of the dried spices to the cream cheese mixture in saucepot, stir in the shredded cheese, and season with salt and pepper to taste.
Combine pasta with squash and corn and the Mexican cheese sauce, then transfer to a casserole and top with cotija or fresco cheese and remaining spice blend. Brown and bubble the mac and cheese under broiler.
Top the casserole with jalapeño peppers, cilantro or parsley, and the juice of 1 lime.