Not sure which one is true, what must I do in this case?

Most of my shipment was damaged at AMZ warehouse, I received reimbursement for all except for 2 units and was told by seller support that reimbursement had been issued for these MISSING units, but then I was informed after a week that the units were DAMAGED even reaching the Amazon warehouse, so I can only receive just 50% of the reimbursement for those units.

How were the units damaged?

Amazon will normally reimburse you for any missing inventory. However, if some units get damaged, they may reimburse you a lesser amount in cases, when they determine that the damage could have been prevented by better packaging, for example.