Does 100% require Concept? If so, where does it go in timeline order?

no, because, by the title of the mission, its a concept, and not a real thing, timeline doesnt include post death, so no killhouse either

100% includes killhouse though?

For 100% you have to do both killhouse and concept

so where does concept go

100% does include Concept. 100% is every mission in the game. There's literally a split specifically when Concept was added lol

It was only added after this post was made as far as I know, and that still doesn't answer where in the timeline I play it.

You play it as last, after killhouse.