ys is clearly not an umpire... I don't think he/she got your first point, Mike, when you tried to lead them to the answer on their own.
The first question is not a bad question.
The second is something every umpire should know, and it seems every coach/parent doesn't. I bet we (umpires) have to correct this 4-5 times a game the first few games of every season.
(Simplified for a non-umpire, with minor details removed) : Below NCAA, a fielder who is in the act of fielding a batted ball has right of way and must be avoided by runners. Other than that, NO fielder can be in a runner's basepath without the ball. Period. If such a fielder causes the runner to deviate or slow from the path THEY want to take, it's obstruction. And note that I didn't say baseLINE - I said path ... meaning the path the runner chooses to take.
And yes ... this means that every time F3 is standing on the bag (even just the inside corner) on a play where the batter rounds (or tries to round) first base, it's obstruction. Every time F5 is standing on 3rd as a runner from 2nd rounds for home (even if she doesn't GO home), it's obstruction.