Although personally not a big fan of either Rüdesheim or Wiesbaden (I prefer Mainz, Bacharach, and Boppard), if your FIL is adamant about Rüdesheim, I would go with the flow.

You will want to stay at the Mainz City Hilton for its convenience to the train station. If for some reason you end up at the Mainz Hilton instead, it is a 15 minute walk to the train station. Alternatively, the number 6 bus referenced below passes very near the Mainz Hilton.

Transportation options from Mainz:

1) K-D is a great idea in the summmertime, but given the time of year, that is out.

2) You could take a train to Bingen and ferry accross, but that tends to take longer (about 1.5 hours on average).

3) Bus/Trains from Mainz to Rüdesheim, changing at Wiesbaden tend to take between 50 minutes and 1:10. Besides the S8 you can also take RB trains to Wiesbaden and transfer there. Alternatively, you can take the number 6 bus, which departs from the platz in front of the train station in Mainz to the Wiesbaden train station, and then transfer to the train.

You should consider day passes (which are valid for up to 5 people) to keep costs down.

Use www.bahn.de to determine which option is best depending when you want to leave.

Transportation from Wiesbaden:

As you indicated, it takes 30 minutes give or take by train.