This feels very Irish to me! I know it is widely used but in the Irish language, 'now' (the word Anois) is used commonly as an interjection, a way of saying the speaker is thinking about what comes next, even that the speaker has finished speaking. It is carried over into English. When my dad was angry, even sentence was ended with 'now.' And it wasn't 'do the dishes now', it was 'I've very disappointed in you now' and 'I expect better, now'. Anyhow - it's not to do with the tense of what the author is saying occured, it is an element of the authors thought process. The story is made of three statements of what occured in the past. Speaking them aloud, the speaker is going through them one by one. First 'even a occured'. now, event b occured. Then even c occured!.

Even though "now" generally means the present, it also has the meaning of "at this point (in the story)". Notice that it comes at the beginning of your examples.

Now can have the usual meaning of time, but it also can be used as a conjunction or as a way to emphasize a point. "Now when I heard this, I was so relieved." <-- used to emphasis a point "Now Alex walked into her first session." <-- this one is not as obvious. In fact, I keep bouncing between conjunction and emphasis. It feels a little awkward.

It is grammatically incorrect, but a lot of informal speech is. The speaker has used "now" in the sense of continuing the story. "Well then" is also used.

here it's used to show the sequence of actions first smth, now (instead of "then"), and now (instead of "finally") usually this is used in TV shows where we witness actions happening a one by one