MacBook Air M3 charging slowly and adapter overheating

Short answer: Return it to your cousin and get your money back.

A bit longer answer: It sounds like you’ve got a few red flags here.

• First off, your MacBook Air M3 should absolutely be charging faster than 18W — the 15" model normally accepts up to 35W–70W depending on the adapter. If it’s only sipping 18W and the adapter itself is overheating, that’s not normal behavior.
• The missing serial number in System Information is another big concern — Macs always report a serial unless the logic board has been replaced or there’s been some tampering. This can sometimes indicate a refurbished unit with non-Apple parts, or worse, a board that was swapped from a donor machine.
• The CMF GaN charger not working correctly on the 65W port but partially charging on the other port also points to either a problem with the MacBook’s charging circuitry (DC-in board, logic board, or battery controller), or possibly a low-quality/third-party cable that can’t handle higher wattage.
• The overheating after just 15–20 minutes while barely gaining charge is a classic sign of resistance in the power path — bad cable, faulty port, or internal hardware fault.