I see a couple of things.
1, the oil under the transmission, most likely came out of the opening where the clutch boot mounts. The thrust bearing that sets inside the boot is lubricated by transmission oil, if the transmission is setting level or front end higher than back end, you are going to leak oil out of that opening. I will say that the trans oil is nasty looking. I've done the same thing myself. Set the transmission to the side and it started leaking oil out of that opening.
2, the oil pattern on the back side of the clutch basket looks to be from the inside of the basket, indicating a failed clutch nut o-ring, or transmission input shaft seal. The backside surface of the middle section(that is rivetied) looks to be dry up to the point that the hub rides on the seal.
I'd probably say that the seal was good, but, as long as you have the bike apart to this point, and if you have a seal, you might as well replace it.
I've not tried to replace the push-rod o-ring that is located internal to the transmission input shaft, I have read of others that have managed to get the old one out and new one in with some small dental tools. As far as the transmission input shaft seal, it might be a good time to replace it if you have no idea on age/mileage of the transmission.