Hi there!
With the hickory you can get away with some violation, with the osage I would chase a ring even though you are going to sinew it. That is just personal though, better safe than sorry.
Regardless of the wood typ I would always degrease before sinewing or any other type of backing. Again, how hard is it to put a rag with acetone over the bow and wait a few minutes?
Can't answer your question about the time between sinew layers but I have read some people wait a week and some people do it all at once.
As far as I know sinew and rawhide will never overpower the belly of osage or hickory.
And why would you apply rawhide over the sinew??? Looks like adding extra mass to me? If you want to protect the sinew from moisture a skakeskin might be a better option?