Shinkansen on Dec 23?

Hello, I�fll be arriving in KIX on Monday, Dec 23. The flight arrives at around 11am. I have a JR pass and I�fd like to take a Shinkansen to Tokyo the same day. Would it be impossible to reserve seats? Also, I�fll be traveling back to Osaka on Dec 27, Friday before New Year holiday. Hopefully a Shinkansen trip before noon. Would it also be difficult to reserve seats? I intend to reserve seats for both trips upon arrival in KIX on Monday. Thank you.

JRP holders cannot book fastest Shinkansen "Nozomi",
book "Hikari" or stpping at all stns slowest "Kodama".
You can book all, also return train seat(s) with at KIX JR-WEST ticket office on 23.
(note)
No shinkansen train fr KIX, so book airport LtdExp "Haruka" until ShinOsaka first.

Vacancy conditions, as posting time.
23 : Y, enough numbers left, but window side "E" seats for viewing Mt.Fuji already full maybe.

27 : Y, but will be crowded than 23, just yet full.

28 : N, start day of year end rush peak, reserved seats already full like this,
https://youtu.be/Z-9XhD1Uo0s
should be lined up for non-reserved for more than an hour.

Bookings on 27th Hikari services are getting heavy - you can check on JR cyberstation - triangle normally indicate less than 20 seats available. You may need to travel non-reserved, which on the Friday should be ok.

I am travelling on the 27th - booked my seats around a month ago.

It may be better to plan to go earlier back to Kansai Region
in the afternoon or evening of Thursday, December 26.

You might hope to reserve your seat
through JR-East Train Reservation website
for the Hokuriku Shinkansen
(Tokyo -- Joetsumyoko -- Kanazawa).
https://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/co...

For your train ride on December 26,
seat reservation via this service should be made
by 11:40 pm of December 23,
and your ticket should be collected
by 9 pm of December 25;
you need to prepare your own valid credit card
(with a magnetic stripe)
of one of the designated brands,
which are currently
VIEW CARD (JR-East brand),
JCB,
VISA,
MasterCard,
American Express,
and Diners Club International.

At the JR Ticket Office in KIX,
you can try to reserve your seats
in Shinkansen trains and other JR Limited Express trains,
including Limited Express Thunderbird
(Kanazawa -- Kyoto -- Shin-Osaka -- Osaka).

Shin-Osaka Station is the only Shinkansen station in Osaka City.
Please note that Shinkansen is not available at Osaka Station.

The Tokaido Shinkansen
(Tokyo -- Nagoya -- Kyoto -- Shin-Osaka),
while it is the quickest way,
is not friendly to Japan Rail Pass travelers.