re: When should a trade be vetoed?

Shouldn't have been vetoed. There are many reasons why someone would accept what you would see as an unfair trade, like if he was burned by Stewart in the past and wanted to take a chance on Crowell, or maybe he's a UGA fan. That trade is at least close so that should not have been vetoed.

Vetoes are for obvious collusion or trades so out of whack that it disrupts the parity of the league. Your example of Julio Jones for Ajayi is a good one. In one league last year I had one owner trying to trade Peyton Manning (drafted in the 4th round) to another owner for Le'Veon Bell (drafted in the 1st round). This was the next day after the draft and they were husband and wife. That's a veto.