My group is flying TSA-HND on a Friday afternoon. We arrive HND around 5PM and need to transfer to Kyoto, but with the following caveats:

1. 3 people have no immigration priority, so the wait could be long - I have APEC, but I can't bring 3 other people through with me, even if they are family (my understanding)
2. we have bags to drop at JAL ABC for transfer to our Tokyo hotel later in the stay
3. we need to take Keikyu over to Shinagawa and transfer to the JR side
4. 2 people are elderly and walk very slow, so we cannot be rushed

If they cannot go through APEC with me, my plan is to go through myself, grab all of the checked bags, and drop off the bags with JAL ABC while the others are still clearing immigration and customs to save time.

So, the question is: should we book Shinkansen tickets in advance and just pad the time until 8PM to make up for any possible delays? Or can we buy the Shinkansen tickets on arrival at Shinagawa? Since it's a Friday evening, I am worried many of the trains will be sold out or it will be be difficult to get seats together or in the same car, which can be a problem, especially for the elderly folks.

If we arrive Shinagawa earlier than planned, maybe 630-700, that leaves time to buy ekiban and relax before the train if we need to buy the 8PM in advance....but I would rather arrive Kyoto as early as possible.