Also, and I am not 100% sure, but I don't think the DSR rating can be changed even if a feedback comment is removed and/or changed??

Only Negative and Neutral FB can be changed by revision request; however once the request is received the buyer can change everything; the colour of the dot (to better), the comment and the stars.  As far as I know there is no option to change positive FB, the option to request revision does not exist. 

ebay can remove the comment from FB left for buyers, not sure if they can also do that for sellers.  eBay staff can only do what the system gives them tools to do; if the option to alter DSR is not there, no matter how many call you make, it is just not going to be possible.

I would not worry, at least it is positive and nobody really reads those.  You cannot be sure that the little chip on your DSR was caused by this buyer.