The 5 bolt rocker pin covers indicate you have a "proper" E model. Ducati spread the use of the ST4 and S4 head mods probably to reduce the production costs. some E's use the old design heads.
Anyway, the parts manuals show you have the right base gasket, ie no o-ring, however that shallow recess in the crankcase looks like the older style using the o-ring seal............
Could be a parts bin confusion.??????
Also, at some point the head gasket design was changed and this made it thinner. To retain the squish dimension the base gasket was thickened up.
I guess what i'm trying to convey is that i'd try and use the earlier base gasket with an o-ring in place, but you must ensure the base gasket remains the same thickness as your current one......................if you went down that route.
Alternatively, you could try and fit a thin o-ring below the base gasket you have now. Using sealant won't give a sufficient seal to the recess in the crankcase IMO.
That oil way is a return from the heads back to the sump, so it's low pressure. It will be hot though and that's why the oil will, if it gets half a chance, escape over the back of your crancases. It being blocked by sealant won't help whatsoever though.
If it were me, i'd go for the solution of using your existing gasket, with not too much sealant in that area and try and get a thin o-ring in there, under the gasket. The sealing face of the cylinder should be flat? If you need any help or maybe a suitable ring, mail me and i'll measure the ones up off the older engines and try and select a suitable one to use. It wouldn't need to be much more than 1mm section i'd have guessed.
Hope that made some sense...................i'm confused anyhow