I agree with all you said:
1) That is how the league rule is written. It is not worded correctly, because sliding can CAUSE contact. But if there is imminent contact, you MUST slide. You cannot try to avoid any other way.
2) I coach and umpire, so I realize there is a huge difference in viewing the game. I am just looking for a rules based opinion, but appreciate you not forcing an answer!!
3) The problem on obstruction usually comes back to league umpires understanding the intent of the rule. For this situation, they got it right one time, and the other said the runner's slide would not have reached the base...that shouldn't matter.
I strive to teach the game to the rules, not to what will get you results in the game. Umpiring is tough, and at some of the younger levels you cannot worry about whether or not each umpire understands all 136 pages of the rule book. If a kid is out by a bad interpretation, then it is what it is. But at the end of the day, they will understand how to play to the rule book and how to give themselves the best chance.