If the countertop swelled, well, your problem is why I made sure my contractor used plywood for underlayment. It's more expensive but a little water won't cause a BIG problem.
If the backsplash swelled, the good news is that you can replace just it. When you do, insist on plywood. Be sure they use a waterproof glue too.
Forget about trying to 'patch' this problem. Once you compress the board, even if you can, it won't last. My best to ya and hope this helps.
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