Two contradicting spells. That's an interesting one.
If encountering such a situation as referee I would say both spells are in effect - as disspelling a spell would be an extra ability and this is not mentioned in the Araby spell ;-).
I'd say "first come first go", so the effects of the bret spell are determined first, means the "missed armour-rolls" are re-rolled - then the araby spell takes effect, so for every hit that did not wound the bretonnian player has to roll again.
2)
Also I have to admit that I never were a friend of this SHIELD OF COMBAT spell. Why the hell does his effect last until the end of the opposing player's following turn (it´s good enough if it is only lasting till the end of the combat phase). ...THIS IS ABSOLUTELY NOT JUSTIFIED...
I disagree. Imo this is indeed justified as you need to first
join the unit with a 45 points wizard/sorceress (a model that does not even add +1 attack if you do not put her on a unicorn, which would rise the prize to 60 pts). After taking that chance you do not even know if the spell works (only a 50:50 chance). Maybe you could take a Wand of Power or Ring of Magic to increase that chance (which would further rise the point prize for another 10 to 30 points.
So it's true, the protection of the spell might still work in the opponent's phase (if the spell succeeded, of course). But destroying that unit (especially if the Shield did not work) is a fat extra-bait...
If the sorceress would not need to join the unit, I would perfectly agree with you. But that's not the case!
You need to see the whole picture before judging...
This spell stands in the tradition of the Undead "Touch of Death" and the Chaos "Boon of Chaos" which all are 4+ and which all need the sorcerer to risk joining a fighting unit.
Greetings,
Gerald