I've been looking at pictures of the head, plug, etc.
It looks to me like that plug maybe can't come out of it's hole far enough to get loose.
I refer to the following (stolen) pictures:
See how close the top of the plug is to the edge of the case (where the breather screen goes).
Notice that the corner of the cover by the breather screen goes over the top of the plug.
I don't think the plug can come out far enough to get out of the hole. The head cover is in the way.
If somebody has theirs "in progress", it would be interesting to know for sure. Put the plug in loose, put the head cover on, see if you can pop the plug completely out by poking it from the bottom.
If it can't come out, then a fix from the bottom without opening the head at all should be OK!