I don't think any aeon overdrives can critical hit, certainly not either of Valefore's. Maybe Oblivion and/or Delta Attack, but none of the others.

Anyway, Focus doesn't pass to the aeon, so it probably was the battles modifier acting here.

As for exactly how focus works, when casting magic, it's treated as if your Magic stat is 1 higher for each time Focus was used (however, this can only accumulate to a maximum of 5 times). When receiving magic damage, it's reduced by 1/15th for first focus, 2/15th after 2nd focus, and so on, up to 5/15ths on the 5th (after this, it has no further effect).

Note that this works slightly different to Shell and Magic Defence+??% (and it works the same as Magic Break). These two work on magic-type damage, whereas Focus works on attacks that use the magic and magic defence stats. While nearly all attacks with one of these properties also have the other, there are some exceptions. For example, Demi and Gravija do magic-type damage, however the magic and magic defence stats are irrelevant to these. Therefore, while Shell and Magic Defence+??% will effect damage from Demi/Gravija, Focus (and Magic Break) will not. Pretty much the only enemy attack which is the other way around(based on magic stats, but not considered magic-type) is Seymour Omnis's Ultima. You'll notice that while Magic Break(if he's even vunerable to it) and Focus will reduce the damage it deals, while Shell and Magic Defence+??% have no effect on it. (Note that this is NOT a regular property of Ultima. This only happens for Seymour Omnis's Ultima, and when using Ultima Fury... maybe Yu Yevon's Ultima as well, not sure on that one). This kind of property is quite rare for "magic" attacks, it's far more common with physical attacks. (However, your party does have a few magic attacks of this kind - specifically, any overdrive which is based on the magic stat - INCLUDING FURIES)