Hmm, I'd say it's borderline -- and open to quite a bit of interpretation: if you use CrMe to imbue a grog with a false memory so he won't appear to be lying when questioned with Breath of the Frozen Lie... does the focus apply or not? Does temporarily suppressing a spell count as countering or not? Does it apply to countering magical effects from magical creatures? Faerie Wizardry?
Good questions some I've thought about, some not.
It's designed to counter, or handle an effect as it is being cast, or at the moment the effect materializes. Ongoing effects are not countered. So in the case of the grog, no, unless he were trying to do it at the moment of casting Frosty Breath, but even then, I'd be no on that. It has to target (small t) the effect, the magic in some specific way, as in fast cast defense.
Magical effects from other creatures and Faerie wizards, I'd not given a lot of consideration, to be honest. I can leave that up to the SG, since he reads the forums. The unraveling spell for undoing other magics is also much harder. The idea is to create a character who can cause other spells to fizzle, or be ineffective. It requires a bit of a generalist approach (which is unusual for someone possessing a focus) and being really fast.
so much of AM is about specific groups' interpretations.
as for me...
a) no, not countering a spell
b) no,
c) if the focus only applies to Hermetic magic, no, if all magic, yes
d) Hermetic Faerie Wizardry of the Merinita variety, yes, even for inner mystery stuff, otherwise as #c
As a rule I tend more and more to apply to myself, I tend to design minor foci that way now:
- Select a verb roughly corresponding to a form (Control, Create, Destroy, Perceive, Transform), and add to that a subset of a form. Like: Create Fire (CrIg minor focus), Dispell Magic (PeVi minor focus), Control Spirits (ReVi), Transform Birds (MuAn), Perceive Water, Create Wood
OR - Select a subset of a form, add another limitation to further restrict it, and apply the focus across all Techniques. Examples based on the previous foci: Magical Fires (Ignem minor focus), Non-Hermetic Magic (Vim Minor Focus), Infernal Spirits (Vim Minor Focus), Birds of Prey (Animal minor focus), Frozen Water, Wooden Wands.
For Major Foci, doing step one but without the verb seems like a good rule of thumb.
I realize some canon foci (like Wood, which is, IMO, too broad for minor) fall out of this, but overall, it works for me as a good guide on foci's width. If something is outside of these, it's a signal I need to take a closer look.
For enchantments, I'm with Ezzelino, especially with Minor Foci. Why would a Flambeau be only able to apply his focus to creating Flames, while a Verditius with a focus in Wood could create, control, destroy wood, and also enchant any Te+Fo into wooden items? This makes minor Foci in material items infinitely better and wider than even Major foci in something immaterial, like Weather or Damage.
Oh, and a note on swords and wooden wands: who's to say these have to be up to scale? Since a sword symbol on a staff is enough to gain S&M bonuses, why couldn't a veridtius with a Sword focus enchant a miniature sword on a pendant?
For spell and foci, I prefer Eric Dahl's approach, though.
It makes everything simpler (no need to recalculate your CT in game), and also means that magi with foci will have more exotic spells: Instead of a spell to create a landed animals, a magus will have a spell to create a dog, another a spell to create a bear... It's, IMO, more flavorful. However, I can't tell if this is RAW or not, and could very well be HR territory.
It's not hard to recalculate. And, it's not hard to list to casting scores, with the larger one obviously being if the focus applies.
By the way, I want to revert my comment that a magical focus applying to all effects that are enchanted into an item that falls under said focus is canon. The application of a focus in this way is only explicitely stated in the familiar section, not the general enchantment rules. While not listed under the explicit differences to the normal enchantment rules, it's the only section where this reference is found at all. This may be "proof by omission" and thus not as indicative as an explicit statement to the contrary, but the application of the focus might actually be justified as "effect based" rather than "medium based", as all effects enchanted can only target the familiar or the magus. Thus it is to be expected that all effects enchanted into the bond relate to the focus. The problematic part of this argument is certainly the magus (he isn't suddenly considered to be of the same type as his familar for the purpose of magical focuses), but given the special nature of the familiar bond, I'm willing to be lenient there. 
My answer would be that Herbam is broader in scope that Ignem, being all living and dead plant matter, wheras Ignem is Fire, heat and light. So flames are pretty much 2/3rds of Ignem, but wood is a much smaller proportion of Herbam. But I am also one of the ones who disagree that enchantment into wood should be of any Te+Fo.
I have weighed in on the Enchantment Focus discussion before, and would have done so earlier in this thread except works bans me from posting on forums from the office 
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I do object to the minor magical focus on enchantment, due to game balance issues. I couldn't believe when I was first told about the Confraternity in HoH:MC that 'Swords' and 'Wooden Wands' were only listed as minor foci. When the player who pointed it out wanted to play a Verditus he made clear he would be making a whole lot of wands, and intended to generate them instead of learning spells. When we discussed the issue, neither of us thought it might mean anything other than a focus in a facet of labwork making wands, rather than a subsection of Herbam as some people in this discussion suggest. I was frankly appalled.
As we all know 'Major Magical Focus' Your magic is much more potent in a fairly limited area, such as weather, necromancy, birds, or emotions. This area should be smaller than a single Art, but may be spread over several Arts - ... You cannot be focused on laboratory activities, although a focus does apply to laboratory activities. 'Minor Magical Focus' Your magic is particularly attuned to some narrow field, such as self-transformation, birds of prey, or healing. In general, the field should be slightly narrower than a single Technique and Form combination, ... You cannot be focused on laboratory activities, such as creating charged items, although a focus does apply to laboratory activities.
In both forms of the virtue 'laboratory activities' are forbidden as the focus, and minor magical focus even clarifies this by specifying creating charged items as a form of lab activity that is banned. I see no reason to think 'creating items using specific shape of wands' would be allowed until HoH:MC came out.
I really don't agree with the concept that narrowing the focus to only a specific type of object should broaden a minor magical focus 'narrower than a single Technique and Form combination' to broaden it out to every Technique and Form, but that is because the text of the Virtue is very specific. If your group want to play with verditus who can be super empowered in item creation then let them, but call it something else.
Diedne Magic is effectively a form of Magic Focus that has been broadened out just as extensively as the bonus some players want to see from magical focus, but it costs effectively 2 major virtues rather than just 1 minor. Now I know some players would argue that Diedne Magic bonus to spontaneous magic is far stronger than item creation bonus would be, but it depends on the player. Some players would take a focus in item creation and only rarely use it, others intend to exploit it to the fullest extent, and one vary specific advantage of creating items is the extra penetration, and creating wands for battle. Spontaneous magic no matter how boosted by Diedne magic virtue is effectively useless in combat against anything with magic resistance, and that makes them more or less level from my point of view.
So let your verditus get their bonus if your group wants them to, but set the right level of payment for such a virtue.