I'd like to add my 2 cents
Veils & Ambushes
For the purpose of an ambush a Veil can replace the stealth roll. In any other regard, the Veil-Ambush works like any run of the mill Stealth-Ambush.
The Veil might still exist after the Ambush, provided there are shifts of duration left. Otherwise it drops. If it stays active It does not however allow for a second ambush attack, or support further attacks in any way.
That brings me to
Ambushes in General
In General Ambushes can only occur once per conflict, namely at the beginning of said conflict. The Mediocre Defense is based on the fact that the target it totally unsuspecting, and that's simply not the case once the conflict has started. Any attempts to "attack from under a veil" during a conflict are best modeled as tags on maneuver aspect like for example "under a veil".
Once an ambush is set you can do nothing else but wait for your shot. This holds true for normal Stealth Ambushes, as well as those from under a Veil. If you do something else than wait, your ambush advantage is lost together with all the aspects created to aid it.
Last but not least
Magical Attacks out of Ambushes
I don't see any reason to disallow it. Where would be the big difference between firing a gun or rifle from ambush and an evocation cast from ambush?
There really isn't one. The Target is still caught unawares, which is what the ambush rules are built on.
As for maneuvers in an ambush to aid in casting. Sure, it's basically just like aiming your rifle, steadying your breathing, adjusting the sights on the gun, putting up little flags to judge the wind... as long as you can think of things to help your spellcasting, you can add aspects just like you can for any other ambush attack.
So Long story short:
1: A Veil can replace the Stealth roll. It does not add additional utility, so no moving ambush or something like that.
2: An Evocation attack acts just like any other attack when used in an ambush.
3: If the Veil has been cast with additional shifts for duration, then it could still exist after the ambush attack. However, it does nothing to aid further attacks.
I think number 3 could use some playtesting to see if it really makes a big difference to allow a block to carry over into the conflict. If it does one can easily say that the Ambush-Attack removes the veil.
That's my take.