The plastic sheet is a moisture barrier - when it rains a lot of water enters the doors from the window, if the sheets are not present or not glued properly specially from the lower side you will get water in the interior and the door panels will crumble away from moisture.
About the locks - if they are not seating with key inserted and you are sure the lock is clean then the lock is not of your car or the key is not of your car. Best is to contact Honda and buy a genuine new key from them which they will get from the chassis no. after verifying your identity and ownership details.
That key should work in all locks - if it does not then your locks need to be reworked to make it work. btw never have a giant keychain with 5 kilos of keys hanging from the key in the ignition, it wears out the lock.