AFAIK if 6 months have passed since the initial repair, the onus falls to yourself to prove that the repair was substandard. More importantly, even if you do go to the time and effort to prove it (and that means expert proof), you aren't entitled for a full refund from the retailer as you've already had some use out of it.
Best bet in these cases usually is the manufacturer, as many will repair/replace from stock without worrying about the devolved value (as they can often make use of the parts from failed units). So it honestly sounds like the retailer is using their experience to get the best solution given the bad situation.