+1 with duruangojo. And + a few additions.
Let's start with the term queso fundido. In practice, it can mean a lot of different things. As a phrase, it just means... wait for it... melted cheese. Despite the similarity in words, It isn't -- necessarily -- the same thing but with different cheese as the Franco/Swiss dish we call fondue; although it can be.
If you heat many (most?) cheeses in a gratin to the point where they melt, and then let them cool on the table -- yes, they'll break, curdle, and/or form a skin. That's one way of making fundido, but not the only one.
"Nachos" are not traditional Mexican food, and we aren't tied to any particular version with roots in the past because it's "authentic."
There are a lot of different ways to make Nachos. One of the simplest is to put a smear of beans on a plate, cover it with a layer of chips, cover that with a layer of grated cheddar, and stick it under a cheese melter. Indeed, that's the version associated with one of the most popular "origin stories." However, it's pretty clear that isn't what Koukouvagia wants. If Koukouvagia wants her cheese to stay liquid, she's either going to have to use a processed cheese, or take some steps to keep it that way. "Regular" cheeses harden when they cool, that's just how it is. She seems to want something more like a cheese sauce.
I don't think making that sort of thing presents much difficulty, nor do I think creating a good flavor profile or texture for chips is very challenging. At least not once you've figured out what you actually want.
If you don't want to use the usual combination of cheddar and American because American is "processed," you can go with a medium or sharp cheddar along with some Jack (to keep the taste from getting chalky and the texture smooth) for something's that's going to taste pretty close -- especially after you've dumped a bunch of salsa or jalapenos in it. Nothing wrong with that. If you want to use packaged, pre-grated "quattro fromaggio" from the supermarket -- why not? Koukouvagia could make a fondue and serve it over a flame with the chips on the side; a Welsh rarebit over chips instead of toast, with still more chips on the side; or just about any other cheese dish/sauce she likes. Who's to say no?
It's just melted cheese and chips.
Edited by boar_d_laze - 6/14/12 at 8:53am