Question one: isn't it time, or isn't it the time?
We always say it's time, not it's the time:
It's time.
It's time I went home.
It's time for you to go to bed.
It's time you started eating right.
It's time for some fun!
We can turn all of these into questions:
Isn't it time?
Isn't it time I went home?
Isn't it time for you to go to bed?
Isn't it time you started eating right?
Isn't it time for some fun?
And yours fits this second pattern:
Isn't it time you stopped acting the goat?
It would be unidiomatic if you inserted the. People might not understand you.
In fact, the article is never used with this sense of the word, which is "the appointed, due, or proper time" (from the OED). Why not? To be honest, I have no idea. As far as I know, it's simply a matter of idiom.
Question two: acting the goat, or acting like the goat?
This is an English idiom, and you can find it in dictionaries:
to behave in a silly way, sometimes in order to make people laugh Insecure and lonely, he resorted to acting the goat to get people's attention.
You're right that acting would normally be followed by like today. But it's not ungrammatical to say it without like, and I suppose when this idiom was formed, it may have been more common to use act this way. But in any case, since it's an idiom, you shouldn't change it by inserting like.
Why do I say it's grammatical? Well, you can act silly, which shows us that act is a verb like be, become, or grow. Since it can take an adjective, we can tell these verbs take "predicative complements" (PCs) rather than objects, like most verbs. (We can tell this because objects can't be adjectives.) So just as you can say being the goat, you can say acting the goat.
Question three: the goat, or a goat?
Again, it's an idiom, so you shouldn't change it. Otherwise people won't understand!
But why the? Presumably it's the same reason we say playing the fool. We're not talking about any actual fool, nor any actual goat. We're referring to the qualities of a generic or archetypical fool or goat.
(Though, to be honest, I have no idea what makes that goat-like. It's a pretty weird idiom!)
Notes
Note one: the subordinate clause you stopped acting the goat is in the past tense, but it doesn't indicate past time. Instead, it indicates counterfactuality. In The Cambridge Grammar of the English Language, this use of the past tense is called the modal preterite.
Note two: Play the goat is a fairly unusual idiom. It was chosen, I think, to translate the original French idiom faire le zouave, which is fairly silly.
Your question is "As far as I know, Past Perfect comes along with Past Simple, but can it be used together with Present Perfect?". The answer depends on where they appear in the sentence.
You have changed "she knew that" from simple perfect to simple present "she knows that". In its new form, it no longer makes any difference to the tenses, as the situation now is the situation that she knows.
The key event in the past is the battle. Looking at the possibilities that you suggested, plus a couple of others, for parts A and B of the rest of the sentence, we have:
A1) many of the men who had followed him into that battle
past perfect simple - valid because it was a completed action before the battle
A2) many of the men who have followed him into that battle
present perfect simple - not valid because it is no longer in force
A3) many of the men who followed him into that battle
past simple - valid because it's still a completed action.
B1) [the men] have never marched back
present perfect simple - valid because it started in the past and continues into the present
B2) [the men] had never marched back
past perfect simple - not valid because it wasn't a completed action before the battle
B3) [the men] never marched back
past simple - valid because, if they were going to come back, they would have done it long before now, so not having done it is a completed action.
B4) [the men] did not march back
past simple - valid (see B3).
You have several choices, then:
A1 works with B1 and B3 (B2 is out anyway and B4 doesn't sound right)
A2 is out, as explained above
A3 works with B1, B3 and B4
Best Answer
In my opinion, your book is assigning a black-and-white tense to every scenario, which isn't how English works. On corner cases like these, either one works. It really doesn't matter which one you use, but in spoken English at least, A) is more common than B). B) sounds a little less conversational, a little more formal.
Of course, if that flight was a month ago, you wouldn't use B). Interestingly, A) can still be used even when the flight is only mostly over!
Situations and what to say: