Why would a wizard use the weakest shift they could?
They would use the largest shield they could for 1 stress (that's what I would do). That way if the attack is greater than the shield strength it soaks up the most amount of damage, and if it stops the attack, it has a greater chance of stopping any other attacks.
Plus, on the player's action, they could choose to extend the shield for longer. Why in the world would someone make a shield for any less than at least their most powerful rote shield?
If someone really wanted to make a crunchy rule for reactive blocks, I think that only allowing a rote shield would be fair.
Who said anything about weakest shift? We're just saying that the decision to throw up a Block is a reaction to being attacked, not a reaction to actually being struck. In fact, given that no additional defense roll is possible, you'd HAVE TO make a decision on whether to throw up a shield before the attack roll to determine which defensive method you're using to protect yourself, either a magical Block or an applicable defense with Athletics/Fists/Weapons.
And I agree, someone can choose to extend it longer, but doing so would follow the usual rules for Prolonging Spells (YS259) and hence cost a standard action instead of being a reactive free action.
As for whether or not a wizard can toss a fireball and then cast an Evocation Block in response to being attacked in the same round, I'd say I'd allow it. The Block in question is more limited than a standard action Block and still costs a stress point, so if the wizard want to defend reflexively, I'd say go for it.
Mind you, as a GM, I'd only allow for a reactive Block like this if the defender was ready for such to happen. If caught totally by surprise, then no go. So basically, anything that would give someone a Mediocre (0) score to his defense roll would prevent an Evocation Block from happening.