like I said... the valve cover gasket is separate from the tube seals, unlike some other manufacturers
blow by gasses is what forces oil past the tube seals into the tube... plus the nearly horizontal position of the head also allows this to happen

there is no chance oil gets past the spark plug from the combustion chamber... not only will the plug eventually back out, you'll have compression loss issues and severe oil consumption if that was the case

this is not a problem specific to the previa, it's a common problem of any OHC engine... it's pretty much always the tube seals or valve cover gasket (if tube seals are part of it)
just spray some brake cleaner down the hole without taking out the plugs... lightly stuff a rag in it while you shoot compressed air into it and it'll vaporize what's being washed off by the brake cleaner and into the rag without leaving a big mess... that is how I did it at the shop on any car

now go get those seals and gasket! ;)