Is it necessary to cook porridge (oatmeal)

Steel cut oats take a lot longer to cook than rolled or quick oats. It's unlikely they will completely cook before the cookie is done. You could try it, but I'd recommend presoaking them in hot water for an hour, or maybe even parboiling them for a few minutes. You'd need to adjust your wet ingredients accordingly as well.

It's definitely asking for rolled oats, not prepared oats. It's just like oatmeal cookies.

It's up to you whether you use normal rolled oats, quick-cooking, or instant. I use normal in my crumbles, but you might choose quick or instant if you think the normal retain too much texture. Personal preference! They'll all work, though, so you can just use what you've got on hand.

(When I do oatmeal for breakfast, I prepare normal rolled oats as if they were instant - just add boiling water. I like some texture.)

And while I'm at it, general advice: crumble topping is something you can easily do without a recipe, just throwing in the ingredients until it holds together a bit, but is nice and crumbly. So don't worry too much about exactness here. It'll come out great.