Learn English – I like / liked the way you act / acted last night

We tend not to speak of having known someone unless there’s a fixed period of time, either explicitly mentioned or implied by the situation (e.g. the person is deceased).

With that in mind, I would switch your two usages to describe the other scenarios, with slight alteration:

It was the first time that I’d seen her since I was in elementary school. A couple of days ago, when I saw her, she acted as if she hadn’t known me (for eight years when we were kids). I think she didn't remember me.

In this case, there’s a specific timeframe in which knowing occurred, which has since ended.

I broke up with her a couple of weeks ago, and we saw each other yesterday. At that time, she acted as if she didn’t know me. She definitely knew who I was, but pretended as if she didn’t know me because she didn't want to talk to me.

This is the common use of the phrase. We think we either know someone or we don’t. It’s like a light switch. She acted like the light was off; she acted like she didn’t know you.

Another common way English speakers would discuss something like this is to say “she acted like we hadn’t met” or “she acted like we’d never met”. These use “met” in the sense that you meet someone and from that point forward you know that person.

Now is an adverb that can mean "happening at or referring the present time or moment."

Now also has another meaning - here's Google's take on it:

2. used, especially in conversation, to draw attention to a particular statement or point in a narrative.

   "now, my first impulse was to run away"

This might be how now was intended to be used above. In this case, it will typically be used in the first word of the sentence and be offset with a comma. Since now would be being used as a "flag of importance" the tense of the verb it modifies wouldn't matter.