As said earlier cosmonauts lived a fairly sheltered and luxurious life so their incentive to defect seems tiny when compared to say, a lowly Soviet seaman on a ship or a soldier at a distant post. Also the Soyuz spacecraft depends greatly on ground control for operations, so a "defection" landing seems very unikely to me. Very interesting scenario though.
It depended on ground control partly
because they were afraid of cosmonauts defecting.
We should also look at the crew of the Soyuz, though. The commander was Aleksei Leonov, who was also the first man to undertake a spacewalk, and was selected for several cancelled Soviet missions including their moonlanding. He was more of a hero than most Soviet cosmonauts, and the next year was made Chief Cosmonaut. Overall, I'd guess he was unlikely to defect.
That leaves the flight engineer, Valerii Kubasov- like Leonov, on his second flight. I can't see a reason why he would defect- though he would have reasons to doubt the reliability of Soviet spacecraft. He was twice scheduled to fly on missions that would have killed or did kill their crew (Soyuz 2, which had the same faulty parachute as Soyuz 1 and ended up being launched unmanned, and Soyuz 11, which depressurized on reentry), and his previous mission had been affected by an electronics failure.
If it did come to a fight between Leonov and Kubasov, I'd guess Leonov would win- he's military and Kubasov isn't, plus he underwent 18 months of intensive zero-G training (unlike Kubasov) in preparation for his spacewalk. But remember, there are also the 3 Americans on board...
Still, I don't think that the Apollo could land safely with an extra man. The only possibility would be to somehow fool the Soyuz's systems into landing it in (say) North Dakota. The US crew were all test pilots, and Kubasov was the only engineer on board. It could possibly be done, but I'd guess that Leonov would have to be co-operating or dead, and instructions would have to be sent up from the US to Kubasov.
The use of Skylab as a place for either an American or the defector to wait temporarily is out- there are no Apollo craft left to send up and get him. Of course, with enough pre-planning (had Kubasov somehow contacted American agents in Russia), that could be changed...