In your pre-game, you should discuss with your partners signals for if you get blocked out or if someone is not able to make a call (Ex. Fair/Foul) that is there's for a very very exceptionally rare reason.
First off, always address a manager by first name, that's how you introduce yourself at the plate meeting, as he introduces himself to you by first name.
In this situation, I do see potential (not saying the call was right or wrong) for how you could change it. Considering that the ball was batted and went out of play, it would be a two base award. Batter-Runner would go to 2nd. You'd have a hell of a time selling a defensive coach on that however. Your "senior district umpire" is incorrect in wanting to give the batter 1B.
After a crew consultation, you make a decision on if your going to change the call or not. If you do you do, if you don't you don't. But if you do, please don't listen to this senior umpire, make sure you get the base award right.
Yours in Baseball,
MLW0038