Are you not allowed to raid lower level dungeons after promotion or are they just stupid? The easy high rank dungeon is basically captured, so grind the mid rank one until you can handle the entry of another high rank dungeon. No one seems to age here so just take your time idiot.
Also seems like the guild actively does not want seekers to succeed. No reason to allow this really even if it's technically legal. It's like when a brat is going I'm not touching you to annoy you on purpose. It's the same outcome, so it ultimately doesn't really matter if they're "technically" in the right. It's also pretty dangerous for the guild to allow a huge alliance form under their watch.