Tacos and Quesadillas

When I cook for more than four people the tortillas are always cold or dry by the time the last person gets their plate. I've tried foil, a warm oven, dry cloth — nothing keeps them warm and pliable for more than 10 minutes. What do restaurants and good home cooks do?

I make quesadillas with chicken or just cheese. Both are good but I want to expand. What proteins or fillings work particularly well in a quesadilla that are maybe less obvious?

Tostadas go soft within about 3 minutes of adding wet toppings. I've tried patting everything dry, adding toppings at the last second, even trying different base tortillas. Nothing keeps them crispy past the point where I want to eat them. Is there a technique that actually works?

My quesadillas are always either burnt outside with cheese not fully melted, or perfectly golden outside but the cheese is still solid in the centre. I cannot find a heat level and timing combination that gets both right simultaneously.

I see the double corn tortilla setup everywhere in traditional taco contexts. But some taquerias use one tortilla and I cannot always tell if that is a style choice or a sign of corner-cutting. Is the double tortilla structural or aesthetic?

I always overfill tacos and they fall apart. But when I try to use less filling they feel like a waste of a tortilla. Is there a weight or a visual reference for the right amount of filling in a standard double corn tortilla taco?