2 man mechanics question

[Daryl H. Long]

Top of 5th inning. Game tied at 2 runs each. Visitors have R1 on 3rd and R2 on 2nd. B1 hits ball sharply past F5 but shortstop makes an excellent stop in grass behind 3rd. Both runners going on hit but then runner from 3rd stops momentarily then heads toward home. Shortstop has three possible plays but given the positions of all runners should have just held the ball. But she does not and decides to throw to first. Throw was wild so two runs scored. After play was over the defensive team appealed that R2 missed third base.
We ruled runner safe and no argument from appealing team thankfully.

Who has responsibility of R2 touching 3rd on the play? PU or U1?

If you were plate umpire would you just declare runner safe or would you confer with your partner since you knew you did not see the touch?

To put the play in perspective I was Plate umpire. At the time of the throw by F6, R1 about 10 feet from plate so I watched her touch then turned to pick up R2, but she had already rounded third and was about halfway home.

The problem results because the base umpire must follow the ball to first. And I as plate umpire have a responsibility to first make sure R1 touches the plate.

IOr is this a situation in which it is impossible for either of us to actually see the runner touch? We declared runner safe but only because we did not actually see her miss the base.