All deals having to go through a merchants' guild. That's actually surprisingly realistic. That's kind of what guilds were about back in the old days. Though that sales restriction would probably not cover craftsmen, as they would be members of their own guilds, which would have permission to sell their own goods. But no idea how that works here.
Yeah, such nonsensical rumours. Nothing for an old man like you.
Ah, yes, throwing water at someone means giving them a cold. Unless the water is literally infected, no.
Sure is running his mouth in the presence of greater people.
Old and senile. Getting what he deserves.
Generally guilds would accept people as apprentices, but those wouldn't be merchants already. An established merchant from another city should be able to get a recommendations easier, such as from a wider range of people. That's the main fault here. Only accepting those who're prominent enough would pretty much rely on the current members for growth, which may not hold up over time, depending on those members.
As you mention, allowing merchants in with restrictions would make it easier to get a foot in, and you can also do some kind of trial system, where they're not full members and don't get all the benefits.