I agree, 10 hours is an overkill. My ATP checkride was long (almost 2 hours) in the Seneca I - 4 approaches, hold, maneuvers, engine shut down and a total of 3 simulated engine failures. DPE said he was looking for complete systems knowledge and sharp instrument flight skills.

As for the challenges you mentioned:
1) Give a full approach briefing and set everything while you talk. If you fly an airplane without any bugs, you won't have to set anything other than frequencies and courses.
2) Get Microsoft flight simulator and practice instrument scan on the six pack round dial gauges or get instrument proficient in a non-G1000 Cessna 172.

I've finished ATP preps in 3-4 hours in our Seneca I. We only have one DPE in our area and since the aircraft doesn't have an NDB or GPS, pilots are limited to ILS, VOR and LOC approaches. This means the checkride will have 2 ILS approaches (one SE), one VOR and one LOC. I have a condensed ATP PTS on my website, if you want to download it. I got tired of reading the ATP PTS over and over again and put everything on one page to review.

If you don't have your written, I suggest Sheppard Air Flight Test 4.0 Prep Software ATP, Flight Engineer, Mil Comp - FAA Airline Transport Pilot. Good luck with your checkride!