When drafting late, which is the better route to go college or high schoolers?

Be contrarian to the rest of the world.

Would the draft select a "looking to be drafted in the first round" if I have the setting set to very conservative?

Your draft reorders based off of your choices then simply goes down the list.  So if you like a guy and redo your choices, you'll have to move him up to be drafted.

The signability issues can be broken down into 3 groups: signable, questionable and doubtful. A "Very Conservative" approach will draft only players that are openly signable. A "Conservative" approach will allow up to 1 questionable selection. A "Balanced" approach will allow up to 1 doubtful selection and up to 2 questionable selections. An "Aggressive" approach will allow up to 2 doubtful selections and unlimited questionable selections. A "very aggressive" approach will not impose any limits.

Right, but you have:

-Will sign for slot money ------------------------------------obviously "openly signable"
-Looking to be drafted in the first X round(s)-----------????
-Undecided.  Still considering college-------------------Questionable
-Probably won't sign-------------------------------------------Doubtful
(might be missing one...I don't have any draft prospects to look at right now)

Is the second one openly signable in the specified rounds, then becomes questionable outside of those rounds?  Or is he considered questionable?  That's the only uncertainty I have.

Yes, if a guy is looking to be drafted in first X rounds, he's considered safe if you're in that range.

I virtually always go "very conservative" in the first round and found a guy a couple seasons ago who signed for over slot (but not much over), so he was most likely a "looking to be drafted in first round" guy.  I think the big monetary difference between a guy like that and a questionable/doubtful guy is that their initial demands are still slot money.