Mike Prince

Yes.

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I would ask him what he saw it if I knew I could have missed it.  Worst thing he can say is no and you live with what you had.
 
I also would not provide a lengthy explanation to the DC when he asked.  Sounds like you are making excuses for not seeing it.  Simply tell him you didn't see it, he may make it easy and ask you if you can get some help.
 
Edit - don't blame the 2-man system for this.  In what you described your partner had nothing to do but see every bit of this play as it unfolded.

The rule book definition is in section 2.00 under "Ball" and "Strike".  It defines a ball as not in the strike zone and not struck at by the batter and a strike as a pitch struck at by the batter (among other things).  But the key here is the term "struck at" which is also commonly referred to as "offering" at a pitch or attempting to hit it.
 
For your situation, it does not matter that the ball was behind the batter or as blue23ll states where the bat was.  He got hit by and did not offer at a pitch outside the strike zone.  HBP.
 
Sounds like you nailed it on the call as well as the EJ.

Yes, the running lane still applies on a D3K, If he runs out of the lane and you have a quality throw he is out.
 
Other runners do not advance on the play, so they are returned to the base they were on at TOP.

Intent doesn't matter, the interference call was correct. A runner can't take away a fielder's opportunity to field a ball.

Don't speak to fans.  Simple as that.  There is never a positive outcome to this.
 
If something needs addressing in your opinion (profanity, personal attacks on you or partner, etc), go through the appropriate head coach or tournament director depending on the situation.