In my admittedly limited experience the 7900 channels do not come back once they are as “clogged” as you say - they seem to actually be damaged, not clogged. And repeated attempts at cleaning them may result in more dead channels. And no, the printer doesn’t give an error, unless you have the auto cleaning option turned on and then it tells you it couldn’t clean it.
But you lucked out in that green channel is extra as far as full color printing is concerned. The printer will run fine if you use the 7890 drivers, but with the 7890 gamut. On a Mac it won’t complain at all, but I think there’s a warning on the Windows drivers. And make sure you turn off the auto cleaning feature in the menu or it will continue to detect the clogs and keep cleaning (and ruin more channels).