Not really, this deal is being looked at from a perspective of protecting consumers choices not protecting Sony's business. Although Sony can have a say the commission are not a mediator between Sony and Microsoft like you are saying. It's the regulators job to make an independent decision using all the facts. Because Sony and Microsoft are the two main players in this (Not sure why Nintendo's stance in this is being side-lined so much but that's another debate entirely) they can make their own case but this isn't/shouldn't be a case of pleasing Sony. From a consumer point of view I would love to see a more even playing field where Sony's current business model is seriously challenged and their position weakened so they have to be more consumer focussed and are not able to command the industry like they do now. If Sony's current business model doesn't work when competing with another party on an equal footing to them then it's upto Sony to change the way they operate. As consumers we will benefit from it.
I really don't understand why this has gone from being about putting the consumers first to being about protecting Sony's huge market share.
At this point in time yes they are the mediator. At this stage Microsoft cannot negotiate deals or make promises for property they do not yet own - the only way anything can be enforced for the future is via the regulators making certain things legally binding.
But like I said previously in this thread, the primary reason for them to be making these offers is in order to satisfy the regulators, not Sony. They can make Sony out to be the bad guy in the media as much as they want but what they think about what is being offered only carries so much weight. From Sony's perspective they most likely won't be entirely satisfied unless the deal is completely blocked.
Regarding "protecting Sony's market share" - I've already discussed this at length in the main thread for this deal but from a regulator's perspective they have a duty to protect the customers of a company who would be negatively effected by an agressive acquisition such as this one.