Batter-Runner Interference?

 
There are no outs, an 0-2 count on the batter (B1), and runners on third base (R3) and first base (R1). R1 is stealing on the pitch. The batter swings and misses the 0-2 pitch, but the catcher drops the pitch.
 
Since first base appears unoccupied, B1 believes that he has not yet been put out, so he runs toward first base. The catcher reacts to B1's actions, throws towards first base but the throw is wild and into right field. R3 scores, R2 ends up at third base and B1 ends up at 2nd base.
 
The offensive coach is delighted with the results of the play. In his view, first base was not occupied at the time the batter struck out, so B1 was entitled to run to first base and the defense failed to put out B1.
 
The defensive coach argues that first base was occupied, so B1 is out on strikes. The coach also thinks B1 interfered by causing the catcher to attempt to retire B1 at first base. He wants B1 called out on strikes, the runner closest to home (R3) called out for B1's interference, and R1 returned to first base.
 
What does the umpire do now?
 
Call
If the third strike is not caught when first base is occupied with less than two outs, the batter is out (NF 7-4-1b). The runners advance at their own risk. In the situation below, at the time of the pitch, the last base that R1 had occupied was first base, so B1 is out on strikes. It is the defense's responsibility to know the circumstances, so play continues. R3's run counts and R2 remains at third base.