unreserved train seats

In late July and early August, I'm going to be traveling by train (but not with a JR rail pass) between Kyoto and Kiso Valley, Kiso Vally and Matsumoto, Kanazawa and Osaka, and finally Osaka and Mt. Koya. I thought to save a bit of money by not getting reserved seats, but I don't understand what happens if the unreserved cars are full. Do they allow people to stand? Sit in the reserved cars? Basically: do you have to leave the train if there are no more seats?

but I don't understand what happens if the unreserved cars are full. Do they allow people to stand?

Yes, people stand.

Sit in the reserved cars?

That is not allowed unless you pay the supplement.

Basically: do you have to leave the train if there are no more seats?

No.

On the first three journeys the reserved seat is only 490 Yen if only using one train on each journey where reserved seating is available.

The reserved seat from Osaka (Namba) to Koyasan is the only option, there are no unreserved seats and there is not a standing option:

http://www.nankaikoya.jp/en/iku/01.html

Thanks

If the unreserved carriages are full can you sit in a vacant reserved seat and pay the difference to the conductor on the train? Thank you David.