And what is a difference between seeing red thing and activation of those neurons? The point of "Mary's room" is to know what seeing red means without actually seeing it.

Depends who's using it. For Dennett, for instance, the point of Mary's room is to point out how ridiculous this notion of qualia is, or at least how silly the thought experiment is.

As stated, she knows everything physical about red. So she knows, for instance, how to build a machine that will activate her red-seeing neurons in the absence of the color. Also as stated, she can perform whatever experiments she needs to in order to become an expert color scientist. So she can have whatever experience would come from having those neurons activated.

If you think there's nothing else to the experience, then I think we're in agreement so far.