Janson's ability takes effect "When you perform an attack". Does this mean after Janson rolls dice, or after the entire attack/defense/alter exchange has been completed and damage has been assigned?

After the whole attack has been completed, so the opposing ship may use any Evade or Focus tokens it has to defend itself against Wes's attack.

The wording on Wes's card is "After you perform an attack..." so all steps must be completed.

I believe this is in the FAQ. The opponent gets to use his tokens during the attack. If he doesn't, it'll be gone by the end of the attack.

Use it or lose it.