Escallion = scallion = green onion.
I think it's possible to substitute regular onions in some situations, but there are a lot of variables.
First off, scallions have a sharper, more grassy flavor than onions, though they're not as strong.
Then there's the question of which kind of onion you want to substitute. White onions have some of that same sharp flavor, and are quite strong. Yellow onions are much milder and sweeter, and might not be such a close flavor match. Red onions are somewhere in between, in my experience. I would probably try white onion, but in much smaller quantity than called for of scallions.
It's also important to consider how the scallions are being used. If they're being cooked into the recipe, that's very different than if they're being added right at the end, or even as a garnish. I think the substitution is less of an issue if the scallions are to be cooked. If they're to be more or less raw, I'd suggest white onion, not too much of it, and sliced very very thinly, as a raw white onion is really strong.
Whatever you try, bear in mind that it won't be exactly the same. But that certainly doesn't mean it'll be bad--just different.
Both crispy and caramelized onions are cooked for a long time, and will be very brown. However, they are cooked slightly differently.
Caramelized onions are usually cross-cut on the onion to release its moisture, and then cooked over very low heat in a crowded pan, stirring infrequently, so that they gradually release their sugars and liquid and it turns to caramel. Depending on the onions and desired result, you may even cover them, an add a little liquid and/or sugar. The end result is very soft and very sweet.
Arabic-style crispy onions are cut pole-to-pole in order to avoid rupturing cells in the onions. They are then fried over medium heat in an uncrowded pan, stirring regularly. This lets them dry out and become brown and crispy, even burning on thin ends. These onions should be a mix of crispy and chewy, and more savory than sweet.
Best Answer
It's a regional preference on what they're called.
When you're buying seeds, they're also called "bunching onions", and I grew up calling them "scallions", although I think that scallion specifically don't have a bulb yet formed, while green/spring onions might.
update : A little research suggests that "spring onion" is the preferred term in the UK, AU and Canada, "green onion" in the US south, and "scallion" in the US northeast, however I'm not sure what the exact boundries are as in the US mid-atlantic, I'll see both "green onions" and "scallions" for sale, with "green onions" being larger (scallions with no bulb, maybe 12"/30cm long, while green onions might have a 2"/5cm bulb, and have over 24"/60cm of green top). It's possible that "green onion" might be a polysemous term that varies by region.