> 1. Global temperature is rising. Any water that used to be above 4°C (or the equivalent for salt water) now takes more volume. But what about water that's below 4°C? Wouldn't that water compress?
It might marginally, but the amount of water compressed will be a lot smaller than the amount of water "uncompressed", both due to the larger range of temperatures above 4°C (below is only a bit, and then ice) and due to the fact that the densest water is at the bottom, meaning that it's easier for everything else to heat up.
> 2. Global temperature is rising. Ice is melting into water. This is new mass entering the ocean. Why can't the sea bed, which is under more pressure due to extra mass, expand?
Expand where? It can't just rise since there is gravity, and the pressure above increases with more water. It can't go down because there is already other stuff there.
In regard to that last point, I hadn't thought of it but there is something called post-glacial rebound, it would make sense that the increased weight on the seabed would deform it, and potentially even cause bulging of land without ocean on top, potentially negating the sea level rise effect to varying degrees worldwide.
It might marginally, but the amount of water compressed will be a lot smaller than the amount of water "uncompressed", both due to the larger range of temperatures above 4°C (below is only a bit, and then ice) and due to the fact that the densest water is at the bottom, meaning that it's easier for everything else to heat up.
> 2. Global temperature is rising. Ice is melting into water. This is new mass entering the ocean. Why can't the sea bed, which is under more pressure due to extra mass, expand?
Expand where? It can't just rise since there is gravity, and the pressure above increases with more water. It can't go down because there is already other stuff there.