Hi, I made a new startup from scratch too, or in better words I “had to” for, just like you, I had high school education long ago, appr.20 years…
I make self study, I use the books suggested in the MCAT page of AAMC website and it works…
You say “…I also recently came into contact with another social worker who chose to do all of her pre-requisites indepndent of an organized pre-med program”… is it possible? I mean dont the US universities demand the prerequisite courses to be taken at an approved US or Canada university? If its possible, then thats great 
Please share whatever you know about it, I really wonder how long this premed takes…