[Europan Ocean (OP)]

If the seabed warms a little sub-seabed methane may turn from solid to gas and bubble to the surface. It would stink, and ships could lose buoyancy. Also much of the Earth's Oxygen comes from sea surface photosynthesis, which could stall with a large amount of heavy methane sitting just on the surface. And the methane would retain heat as a greenhouse gas.

How likely is this? How accurate?

The world cannot maintain Oxygen supplies without photosynthesis.

Does CO2 in the upper atmosphere reflect heat energy away? Or it only retains heat?

[Bored chemist]

large amount of heavy methane

Methane is less dense than air.

[chiralSPO]

methane doesn't have an odor (we add sulfur-containing compounds to natural gas so we can smell it)

it wouldn't accumulate at the surface (as bored chemist points out, methane is considerably lighter than air), and even if it did, is unlikely to interfere with marine photosynthesis (it i isn't going to prevent water or carbon dioxide or light from getting to the chloroplasts)

methane  IS a potent greenhouse gas, and this is one of the more concerning vicious cycles associated with climate change: warmer temperatures means lower stability of methane clathrates, means more methane released, means warmer temperatures...

[Europan Ocean (OP)]

Gas from the seabed would carry a smell wouldn't it?

And CO2 in the upper atmosphere, does it reflect or retain heat?

[chiralSPO]

Gas from the seabed would carry a smell wouldn't it?

It might smell like the sea...

And CO2 in the upper atmosphere, does it reflect or retain heat?

Both carbon dioxide (CO₂) and methane (CH₄) will mostly lead to heat retention when in the upper atmosphere. Both will slow the transit of infrared radiation (IR), and this applies to IR going away from the Earth as well as IR going to the Earth. But, most of the energy that the Earth receives from the sun is not IR,  but visible light (which is unaffected), while most of the energy radiated by the earth is IR. Thus the net effect of slowing the passage of IR through the atmosphere leads to an increase in the temp of the earth.