>> I plan on reserving a single compartment on the Sunrise.

You can forget about that one.

Nobinobi seat, maybe. But it's torturous.

You can check seat vacancy online, but only in Japanese

http://www.jr.cyberstation.ne.jp/vacancy/Vacancy.html

Select date/date, then select the last option 在来線列車 of the drop down for " non shinkansen trains"