Couldn't a "creature-less" set simply have spells that just create creature tokens as stand-ins? Cards like Call of the Conclave in place of any creature that would've been in the set, even for Legendary creatures?

If they’re normally tokens, they’re mostly vanillas (no rules text) with a few French vanillas (just creature keywords). That’s not enough texture to make combat robust enough. If the tokens are more complex than that, then why aren’t you just making creature cards?

This article (ask-wizards-may-2007-2007-05-01) mentions that instants and sorceries only make creature tokens when there's some reason it couldn't be a creature card. For example it has some ability like flashback, it makes more than one creature, or it makes a type that isn't put on cards for flavor reasons. This logic doesn't seem to cover a lot of cards though, like Call of the Conclave or Heat Shimmer. Why were those cards tokens instead of cards?

Call of the Conclave was in a guild where the guild mechanic was Proliferate (a mechanic that copied creature tokens). My best guess on Heat Shimmer was the token allowed for a cleaner template. It also avoided the problem of figuring out what creature type a Red Clone is.

Why is it that Call of the Conclave makes a monocolored token instead of a multicolored token?

We have a limited number of tokens made for each set based on the fact that we now produce tokens on the backside of the ad cards. The only 3/3 token for Return to Ravnica was slated to be a 3/3 green centaur so that’s what Call of the Conclave ended up making. It was decided that it wasn’t necessary to eat up a token slot making a green and white 3/3 when the monogreen one would work fine.