Let's talk obstruction rule ASA

[Steve M]

Don,
For your situation 1, it's going to depend somewhat on whether the players had to to prevent the contact AND on just how far did the fielder have to go to get the dropped ball. I'll give the fielder room enough to make a reach or a step but probably not both. That's pure judgement, nobody is going to be able to argue with you on whatever you decide to call, but you may have both sides trying to argue.
Now, your variation has the runner doing something to intentionally make contact. That makes this an interference call. When defense intentionally moves to have an impact on the offense, I've got obstruction. If the offense intentionally does something to hinder the defense, I've got interference.
And for your last one, with the fielder knocking the runenr down and the runner deciding not to advance, the case book shows that you make the base award anyway - when the play is done. See Play 8.6-7.