Your question asks whether the addition of でしょう changes the assertion of a rhetorical sentence from positive to negative. In this case the simple answer is "no" but perhaps your real question (which you might want to revise) is:
"I understand sentence endings that include 〜のではないか are positive but is this always the case? eg if they include でしょう？"
If I follow your application of logic then two "+ve" s => +ve result
But, you don't have to rely on this logic. A better approach is to ask yourself:
How can I be sure 〜のではないだろうか is positive? After a while this becomes intuitive but two simple checks are:
What is the context? (the most useful check but not always something we can be confident of understanding when we are stil learning)
Are there any other useful words that confirm the meaning? (eg おそらく or もしかえしたら）
The following examples/explanations from my notes might help:
1. Typical example
I was worried he might have banged his head when he fell over.
2. Conversational example
= I think / probably. おそらく is often used with:
4. もしかしたら / もしかすると
which often comes with: