Question about a scenario about going from enlisted army national guard to active duty army OCS?

The National Guard has it's own OCS, though, with what you mentioned ROTC sounds like a better option. You need at least 90 college credits for Active Army OCS (in other words, 1 academic year short of graduating) and you must complete the academic requirements by the time you make O-3 i.e. 36 months/3 years.