I do like the quiet rooms idea or more precisely the ability to change access, perhaps there could be an "Intercom System" room where knights (of a rank determined by the GM) can use a machine that allows them to select who hears what they say. Simple checkboxes of what rank guildmates can hear and what locations broadcast to would allow guildmates to address all the officers in Haven, the recruits in the clockworks, or the Veterans in the guildhall.

Getting into a guild is easy, you can literally stand in Haven on a new account and you will eventually get an invite. To make it more helpful and accessible this could be one of the cheaper guild upgrades. That combined with the fact that selling minerals generates CR means that endgame knights in the guild can choose to vendor their minerals and donate to the guild.
Locking the Gate deposit would allow guilds more control over what they become, so it makes sense; last thing you want when constructing a gate is for someone not in your guild to just empty all his/her minerals and screw up the gate you have in development.

I don't know about the Game masters opening their guildhall, but they could perhaps make an alternate one where they can mingle with players and share news about upcoming developments.