JR Kansai Hiroshima pass during New Years?

Hi,

I would like to gather your thoughts on whether I should buy a JR Kansai Hiroshima pass for my travel during the new year holiday. There is definite cost savings, but they are not worth it if I am not going to get a seat on the trains.

Here is my understanding (feel free to correct me):
-the JR Kansai Hiroshima pass does not include reserved seating. -if you buy your shinkansen tickets online rather than use the pass, you can reserve seats online up to 1 month in advance. -starting December 28, many Japanese will be traveling for the holidays, and reserved seats fill up very quickly, so it is recommended to reserve seats ASAP.

Here is my general itinerary:
Dec 26 - arrive at KIX at night, JR office is still open if I remember correctly, but a 1 or 2 hour delay will change that
Dec 27 - USJ all day
Dec 28 - relocate from Osaka hotel to Kyoto Airbnb, probably go to the aquarium
Dec 29 - day trip to Hiroshima (peace museum's last day of the year) and Miyajima
Dec 30 - day trip to Nara
Dec 31 - Kyoto shrines and temples
Jan 1 - Kyoto shrines and temples
Jan 2 - Fukubukuro and fly home via KIX

Questions:
1. If I buy the JR Kansai Hiroshima pass before arrival and if I can get to a JR office on Dec 26 at KIX, will I be likely to get nonsmoking reserved seats for my Hiroshima day trip on the 29th? What about if I attempt to get reserved seats on the 28th?
2. If I do have the opportunity to buy reserved seats, about how much is the surcharge per way?

If it's too risky that I will not get reserved seats for the Hiroshima day trip, I would probably pay for regular tickets and not get the pass. It's just my husband and me traveling, and I refuse to be in a smoking car (I had to do this before when we didn't reserve seats on time, and it was torture).

Please feel free to comment on anything else about my itinerary.

Thank you!