Oneway Bullettrain - online reservation

How do we buy and reserve seats online (from home) to bullettrain, when not buying a Japan Railpass?

We are 4 pax going from Tokyo-Hiroshima-Osaka-Kyoto-Nagano-Tokyo for about 2 weeks and then one week in Tokyo afterwards . The Railpass calculator showed that it's not recommended to buy 14-days railpass, but it seems impossible to reserve seats from home without the railpass.

We will be going Tokyo- Hiroshima on March 30th, and read somewhere that seat reservation at that specific time will be necessary if we don't want to stand up for 4 hours. Is that true?

It is explained here:
https://www.japan-guide.com/e/e2018.html#ticket

We will be going Tokyo- Hiroshima on March 30th, and read somewhere that seat reservation at that specific time will be necessary if we don't want to stand up for 4 hours. Is that true?

No, it is not true. You could make sure to get a seat in the non-reserved car at Tokyo Station by going to the paltform much in advance and line up. Even if you couldn't find a seat, you would likely be able to catch one at a station before Hiroshima. However, traveling with seat reservations is much more comfortable.

Correct, a rail pass would not be a good idea. Best for tickets is either buy online from the official site of from a ticket office. The JR staff will make sure you get the benefit of the combined Hiroshima to Tokyo trip as a single base fare ticket covering the intermediate stops (makes things cheaper). For Kyoto to Nagano you have a choice of either via Kanazawa (you can include an afternoon there), or via Nagoya and the Shinano express (an inland route, might be slightly slower, but roughly the same travel time).

As Uji and Japancustomtours said, you can also just buy a shinksansen ticket directly at the station if you really needed to. You can get either a reserved seat or non-reserved.