Hi Luca. I expect Simon will be along soon to give the official interpretation, but here is my understanding.
Knights can't evade. Because of the way units are classified, knights are not cavalry (the definitions on pages 12 to 25 are mutually exclusive) and so are not in the list on page 43 of units that can evade.
If the crusaders had been cavalry, rather than knights, then it gets a bit complicated. As I understand it, the charge is a flank one, and so can target either of the crusader units. If it targets the rear one, that unit isn't completely pinned by the ZOC as it is behind a formed friendly unit, relative to the Ayubbid cavalry. It can move sideways as well as directly backwards. However, the only possible evades are backwards or diagonally, since the charge is itself diagonal. I don't think the diagonal evade is allowed, as an evade must be into a box the evader can 'legitimately' enter and it cannot legitimately move diagonally. So if it evades, it must go directly backwards. If, on the other hand, the charge is directed onto the front of the two crusader units, I don't think that unit can evade. It can't evade away from the charge along the diagonal, because it is in a ZOC and such a diagonal move isn't a 'legitimate' one. It can't evade backwards, because it cannot interpenetrate the friendly unit behind it (cavalry can't interpenetrate cavalry). So it has no legitimate evasion route.
My interpretation is based on the idea that the requirement that the evasion is into a box the evader can legitimately enter excludes evasion movement that would be prohibited by other rules, such as the ZOC or interpenetration rules. I would be interested to see if others agree.
The rally penalty applies for any unit that can charge. Infantry have to be in an adjacent box (including diagonals). Cavalry may be in an adjacent box or must be able to move one box straight or diagonally, then charge a unit straight ahead in an an adjacent box. Note that the wording on page 42 apparently excludes a cavalry charge that is a forward movement followed by a diagonal charge.