The core issue is “does the charger conform to MFi standards?” If it does, Apple issue. If not, Belkin issue. With the evidence presented, balance of probabilities would point to Belkin (as no other MFi chargers appear to have the same issue). From what I understand, MFi is a self certified process, so Belkin can state “MFi” on a product without any form of independent verification.
If MFi certification includes voltage/charging standards (have not verified this) then it could be argued that Belkin failed to adhere to the technical requirements. That would then suggest that Belkin misrepresented the MFi compatibility. Misrepresentation = refund !