1) Would it be allowed to use this “reactive evocation block” in an exchange that you have already acted? (I would say no)
The whole point of this is to help keep your squishy wizard PCs alive. The main limiter on the use of evocations is in the mental stress involved, so might as well be generous in how and when evocations can be used.
So, I allow one of the more generous interpretations of reactive blocks:
Generating them is a non-action
Unless they are broken, they last until the end of your next action, which may mean that your block persists into the next exchange
2) In case the block is overcome, the rules say that there is no other defense roll allowed (“no defense aside from that”). Would that mean that the difficulty to attack a target (whos block has been bypassed) will be Mediocre (+0)? (I would say yes)
No, the block subtracts damage as normal. But under 'normal' evocation blocks, you can roll your defensive skill (normally athletics) and take whichever result is higher, you defensive skill roll or the evocation block you are under.
Note that this rule also prevents you from making the baddies easier to hit by 'protecting' them with a very low power evocation block.
3) Would it be possible to do a reactive evocation block by using an enchanted item (designed to create a defensive evocation block)? (I would say yes)
Yes, activating an enchanted item is a non-action. You can even activate two different defensive items against the same attack, using one to provide a block and the other to provide armor. (In addition, remember to roll your defense first, since if you get a +4 result on the fudge dice, you might end up with a better result on your defensive skill than the block provided by your magic item.)
4) The same question as 1) above, but done by activating the enchanted item from 3) - Would activating the enchanted item reactively (once you are attacked) use up your action in the current exchange (i.e. you can't do this in case you have already acted this exchange)?
No, activating defensive items is a non-action.
In addition, any defensive magic item can act as block or armor when activated. Though note that once activated, the chosen type of defense persists, so you can't change from a block to armor without paying for another item activation.