Just look at wikipedia: https://en.wikipedia.org/wiki/Global_warming_potential

It is always important to look at the time scale. Methane has a lifetime of 12 years. This means for a a longer time frame (most of the time we are looking at a 100 year time scale), the effect of methane shrinks as it does nothing for 88 years, but a lot in 12 years.

Yeah should be approximately 0 K. The amount of radiation is a function of the bodies temperature.

Most of the time when you expand via a valve in a refrigerant cycle you expand into the two phase region from a subcooled state. Expanding a liquid does not give us a lot of energy back. With co2 as a refrigerant there are ideas to use an expander to raise the efficiency of the cycle. But that's a special case as co2 has a flat saturation curve where we have a high vapor content while expanding. Also, expanding while having liquid content in the fluid can damage most expanders.

Ok, you want to charge the ship via a landline and when it is away from the harbor, it only runs on its thermal batteries so to speak? We could do the math, but there are two problems: you would need huge storages for the energy (it does not matter if you store the energy in batteries or as steam).

Ok I do the math quick and dirty: A big ship has an engine with 37 MW (megawatt) power (Jahre Viking). You can store 0,2 kWh (kilowatt) per kilo steam (at 100 bar at 500 °C), which means you would need 185 tons of steam an hour to drive your ship. When you are like four weeks at sea, you need 124.000 tons of storage for your steam.