The cgs you're missing on page 12 are indeed from Oki and Okan from the the party, you need to go out and gift them (The Watch for Oki and the Wet Summer magazine for Okan) everyday since they become available at the spa to be able to invite them, I did so it's possible.
The last cg of the 2 row of page 20 it's John, the white haired guy at the hospital, to get him you have to do Kovit'so route, having s*x at the hospital and the next day talk to John a couple of times, I did it this way (I'm still missing the Dr cg tho).
Last 2 cgs of page 18 and page 19 are for the After Story and we currently can't get those, we need to wait for an update in the future.
Hope it helps!