Firstly, yes, if a spell requires an attack, that attack still counts as an attack.
So, what you have to remember is the "specific beats general" rule. This is detailed on page 7 of the PHB. The core of the rule is:
If a specific rule contradicts a general rule, the specific rule wins.
So the general rule here is that familiars can't attack. There are 2 exceptions to this general rule:
A familiar can deliver spells with a range of touch on your behalf, even if the spell requires an attack roll.
A Warlock with the Pact of the Chain can forgo one of their attacks to let their familiar make one.
So when you cast a spell with a range of touch, you can have your familiar do the actual "touching" on your behalf. You cast the spell as normal using whatever actions it requires. The familiar is required to use their reaction, and if the spell requires an attack roll, it makes the attack roll. Since it uses your modifier anyway, it's exactly the same as you making the attack roll except that you don't have to stand next to what you're casting the spell on.
In the case of a Warlock with the Pact of the Chain, the Warlock takes the Attack action as usual, then has the familiar do the actual attack. The errata for the PHB says:
When you
let your familiar attack, it does so with its
reaction.
If the Warlock can make multiple attacks with the Attack action, the familiar can use its reaction to replace one of them with its own attack, then the Warlock makes the rest.
Finally, only spells with a range of touch can be delivered by your familiar. There is no option for having your familiar cast a spell on your behalf, and the option for allowing a familiar to attack instead of yourself specifically says "when you take the Attack action", not just "when you make an attack".
No.
Consider that:
Verbal (V)
...the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion.
If you can argue that this is ALL that is needed to cast a purely verbal spell, then anyone or anything with a mouth can cast it.
Let's get more basic: suppose you don't have spell slots left to cast Healing Word. Will speaking the words that produce "the particular combination of sounds, with specific pitch and resonance" set the magic in motion?
It all boils down to the power to cast it, and I'm afraid your familiar does not have it.
Let's look more closely, then, at:
Voice of the Chain Master
Prerequisite: Pact of the Chain feature
You can communicate telepathically with your familiar and perceive through your familiar’s senses as long as you are on the same plane of existence. Additionally, while perceiving through your familiar’s senses, you can also speak through your familiar in your own voice, even if your familiar is normally incapable of speech.
There is nothing in there that ever mentioned -even vaguely- casting of any sort.
The argument of specific-beats-general here is also invalid because "specific" also denotes "explicit" and there is nothing here that explicitly and specifically grants you the power to cast verbal spells through the familiar. This case is more of a general-beats-vague.
Finally, the Find Familiar spell states the times when a familiar can (sort of) cast a spell:
Finally, when you Cast a Spell with a range of touch, your familiar can deliver the spell as if it had cast the spell.
Voice of the Chain Master lacks similar text that lets you cast verbal spells through it, so the feature can't let you do that.
Best Answer
It is likely that only one familiar would be able to attack
There are a few things that point to this being the case:
The Pact of the Chain feature uses the singular
The Pact of the Chain feature states:
Though this feature existed before the flock of familiars spell, the feature didn't receive any errata with the release of Lost Laboratory of Kwalish. Applying the feature as is it consistently uses the singular throughout, which, strictly Rules As Written, means that only one familiar can gain this benefit at a time.
Other parts of the flock of familiars spell only apply to one familiar at a time
The flock of familiars spell states:
This is more of a supporting idea; here we see another feature of familiars applying only to one at a time so it would make sense for other features to work similarly.
Note also that flock of familiars never states that the familiars function as some sort of collective unit instead each individual familiar follows the rules of the find familiar spell.
If this were allowed, other familiar capabilities would be able to be done in unison
If all the familiars could attack in unison, then you could also dismiss and recall the familiars all at once; both of these abilities use the singular in their descriptions:
So if Pact of the Chain's attack worked with all the familiars at once, so would this ability. Though it's not game-breaking by any means, suddenly summoning between three and ten familiars is certainly a significant buff to your party. This somewhat points towards Pact of the Chain's attack only working with one familiar at a time.
Arguments 2 and 3 are much weaker than argument 1, especially since the flock of familiars spell is somewhat inconsistent: It specifically says touch spells can only be done by one familiar at a time, yet it also says:
And doesn't mention whether this sense-sharing can only apply to one familiar at a time, or even whether it is always on (the version in find familiar requires your action to activate and removes your own senses while sharing). And then flock of familiars doesn't mention the ability to dismiss/recall familiars at all.
The spell isn't consistent in how it addresses the abilities of find familiar and how they change (or don't change) with the flock of familiars spell. As such arguments 2 and 3 are significantly weakened, though argument 1 still stands.
Overall, the use of the singular in the Pact of the Chain feature and the fact that some (though not all) features of the spell are called out as working with only one familiar at a time both support the idea that Pact of the Chain would only work with one familiar at a time.
Note also that Pact of the Chain is not itself part of the find familiar spell, and also Jeremy Crawford, though he was a designer of the book, was not the lead designer like he was for other books for fifth edition. It is quite possible the Pact of the Chain feature was forgotten about when writing the description for the flock of familiars spell.
A small note:
The Pact of the Chain features states:
You do not forgo your entire Attack action, just one attack that you can make with the Attack action (features such as Extra Attack allow you to make multiple attacks with the Attack action; you could then forgo each attack individually as you wished). For further reading on the distinction between attacks and the Attack action you can read: "What does upper-case-A-Attack action vs. lower-case-a-attack mean?"