So what method would be the best for a would-be cow bowhunter?

Spot-and-Stalk?
Stands set up in high-traffic areas?
Baiting?
Shoot them from a vehicle?
Cattle Drives?

And I'd guess you'd want to be using a high-KE setup with a cut-on-contact head.

Do you think the other cattle would "wise up" after the first one went down?

Personally, I'd start off with a spot-and-stalk technique, trying to keep the shots under 25 yds. If that didn't work, I'd be willing to explore other avenues.