1) Yes.
2) No. Star Rating from what i understand is just on the players skills and has nothing to do with potential. The Star Rating tells you whether your playing will have an A grade rating (Meaning A-, A, or A+) or a B grade rating (Meaning B-, B, or B+), which would be shown as a 4 star star rating and so on.
3) Yes.
4) I think it's a toss up between the two 18 year olds and would depend on what positions they are and what their heights are if you would want to sell the draftee you get, and what your long term goal is for the player you get.. (Do you want to train them to be a 20-35k player or do you want to train him to be a 40-50k+ player?). I think in your current league and looking at your roster all you really need is a 3 star potential guy and it can get the job done for you. Personally I would set my draft order like this:
1st: 5 3 A+ 18 (Assuming he has a good height for his position)
2nd: 4 4 B+ 18
3rd: 5 4 A+ 19
4th-48th: 5 3 A- 19 (And to be honest i would probably put blank players before this guy just because i know he is already 19 and doesn't have an A or A+ rating or really good potential)
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs