Steve was going to leave Namor BEFORE they found he was affected by the Negative Zone. He wanted to get Hope and Cyke out.
In Agents of Atlas it's revealed there is an new hidden Atlantean city in the Pacific called Oceanus. In the Defenders it was revealed they are clearing an old site for a new capitol. In his own ongoing, they talk about several smaller settlements like the Southern Reef where the Tridents hail from. And IIRC, the bulk of the Altantean army is stationed elsewhere, but I can't recall exactly if it was named.
True, but his alliance with the X-Men did give him New Atlantis ... right off the coast of the US. The X-Men did help when Osborn weaponized Marrina and sent her after the Atlanteans.