Excellent -- good work.
So let's look at the messages you got originally:
You can see from this:
UNIQUE KEY `topic` (`ID_TOPIC`,`ID_MSG`),
UNIQUE KEY `ID_BOARD` (`ID_BOARD`,`ID_MSG`),
UNIQUE KEY `ID_MEMBER` (`ID_MEMBER`,`ID_MSG`),
KEY `ipIndex` (`posterIP`(15),`ID_TOPIC`),
KEY `participation` (`ID_MEMBER`,`ID_TOPIC`),
KEY `showPosts` (`ID_MEMBER`,`ID_BOARD`),
KEY `ID_TOPIC` (`ID_TOPIC`)
That the `ID_TOPIC` key is already contained in the `topic` key -- hence it's redundant.
As for the id_member field it's complaining about, all the keys are different two-column indexes, so that's fine; and is the cause of the 3rd and final message.
Personally, since it's third-party software, I'd contact them, and do nothing about it. But if you really want, you can drop the last key.