Okay, so to start, I'm not an "AC person," generally. I know the basics and don't have any problem doing those things (running wires is always fun
), but if things are outside the box...
But, I did think about using something like a source selector switch. These are high-rated rotary switches that you use to choose an incoming AC source (shore vs. gen, for example). You still have to have breakers elsewhere though, and they are expensive, so I'm not sure there's a good reason to re-purpose one as a "load selector" switch.
And I don't think just jamming two sets of wires into one breaker is probably a good way to do it. I also wouldn't use the fairly-heavily-loaded outlets circuit.
So I think probably the way to go is to add a breaker. Then, as you say, you can just run a wire up along the wall behind the couch (appropriately loomed/secured) to the water heater. I don't have my brown box to look at anymore, but I did just help a friend remove his and it was the same type as ours and did have five AC breakers (we have four). The fifth one was for the electric water heater, in fact. So perhaps there is expansion room in the brown boxes in general. This would likely also be much less expensive than a source selector type switch, and more correct anyway.
You'd still have to manage your 30 amp total, but both the microwave and water heater circuits would be legit.
Generally 14 gauge wire is good for 15 amps and 12 gauge for 20 in the AC world, from what I know.
If you decide to do this and want some intel on how the brown box comes out/is wired, just say the word.