The 12 years are added to a two-year sentence Sam Rainsy is already facing for uprooting markers along the Vietnamese border last year.
If the penal code goes ahead as it stands, it will have ignored concerns from civil society.
In his report, Subedi urged more tolerance of criticism by public figures and cautioned against using the courts to silence dissent.
Sam Rainsy remains in exile abroad and was not present for the hearing, which lasted from 8 am to 1:30 pm amid heightened security.
The general strike, which will hit Cambodia's main economic earner, is scheduled for Sept. 13 through Sept. 18.
Villagers in Kampong Speu province have alleged that sugar plantations operated by Ly Yong Phat have pushed them off their land.
The Cambodian government is facing more accusations it has cracked down on freedoms of expression and the media in the past years.