Apparently my gas mileage has collapsed because of a sticking valve not rotating in the valve guide as it reciprocates.

Can anything "simple" resolve this without disassembling the engine?

 

That's not how it works, yes valves do continually shift and rotate (hence why you lap valves) but it's not something that is controlled. Who told you this and what makes them thing it's in ANY WAY related to fuel economy...

Usually a sticking valve is due to damaged guides, you also usually have a puff of blue smoke from the tailpipe when you first start the engine in the morning (but not blue smoke at high rpm or under load which usually means piston rings).

You can also have a sucked valve that is physically damaged but that's extremely uncommon on a stock motor.