Can a soldier take their JST straight to S1 and have the credit hours added to their civilian education, without being enrolled in a college?

No. It needs to be evaluated by a university. The JST only provides suggestions insofar as assigning credit hours to particular "courses". Many of the items on your JST will never actually translate into college credits.

As SGT Joseph Gunderson said, have the soldier enroll. It doesn't have to be a major school, but just the local community college, they'll evaluate the JST. The soldier then can knock out the rest of the initial "core" classes - 2 writing, 2 sciences (w/ lab), a math or 2, 2 humanities. The service will knock out lots of electives, but not much of major requirements save for a few specific MOS/majors (and this is where scouts/infantry really suck is translating to actual civilian stuff). A JST will also list "military science" or "leadership" along with every major course (NCOES/MOS), the school won't credit it more than once.
Yourself and every Soldier you can influence should be knocking out these simple courses as early as possible - I wish I did mine earlier. Yes gunnery and the field make it hard, but maybe a group of you (could save a lot of money on books) all get in the same one and instead of usual fri night barracks parties, you have fri night school. The courses really aren't hard when you look at them individually, it took me a while to figure out how actually easy college courses can be.
Influencing this could be a great way for SPC McCormick to become SGT McCormick as well - not that you should do things for the sole purpose of getting promoted.