Wandering Trader Spawning?

The wiki says that every 20 minutes the wandering trader has a 7.5% chance of spawning within 48 blocks of the player, but:

• Does the normal "not within 24 blocks of spawn point" rule apply?

• Does he only spawn on opaque, full blocks?

• Can the odds be increased by making multiple layers he can spawn on?

• How is it determined whether the player is "underground"?

• How does it work differently in multiplayer? The wiki just says "the player"

• Does the normal "not within 24 blocks of spawn point" rule apply?
  Yes.
• Does he only spawn on opaque, full blocks?
  Yes.
• Can the odds be increased by making multiple layers he can spawn on?
  No.
• How is it determined whether the player is "underground"?
  This doesn't have relevance, but it's based on a mixture of factors which I don't know. I believe it's being inside the "cave" generation chunks.
• How does it work differently in multiplayer? The wiki just says "the player".
  It picks a random player to spawn it near.

Wandering traders can spawn anywhere within 48 blocks of the player. I once had one spawn about 3 blocks away from me.