I think your correct in your assessment about oil on the clutch. My suspicion is that changing the oil isn't the culprit but the long rest (10 years) had the oil pretty gummed up and the new oil just provided the detergent properties to get in all those nooks and crannies and get the leak started. You may need to change the rear seal to cure it but pulling the pan would allow you to clean it out and get rid of all the crud that has accumulated over the years. Long sleeps are often tough on engines. Sort of like an empty house that starts to fall apart.
As for the shifting it is likely all those worn parts causing the problem. At first when I read your statement about slipping out of gear I thought "ah-ha...synchronizers" but then you stated it only happens while cruising and my experience is they jump out when you let off the gas so I doubt it now. That leads me to think it's the worn shift linkage with a little vibration that they all seem to have.
