Curious about the phrase "しまいと" in a book I'm reading

Question

I'm re-reading/translating a section of 金原ひとみ's 星へ落ちる for a translation course I'm taking and ran into the following sentence:

彼がこの家にいない時、私は彼があの人と食事しているところとあの人とセックスしているところを想像しまいと、延々タバコを吸っては自分を無意味なものへ変えていく。

I've never run into this usage of しまい before and am curious about why it's being used here. A quick web search brings up mostly the phrase 「しようとしまいと」, so I've parsed it in this sentence as the equivalent of 「しない」, meaning that it's the -まい form of する and making the sentence read as something along the lines of "When he isn't home, if I don't imagine him sharing a meal or having sex with that person, I endlessly smoke cigarettes and drift into a meaningless existence."

So I guess my questions are:

1. Did I parse this sentence correctly?
2. Is there any particular reason for this usage of -まい? The novel consists mostly of a woman's inner dialogue and isn't at all formal, so this felt a little out of place.

Answer 1

You can forget the (よ)うと～まいと～ pattern described here for now. This まい simply expresses a negative volition ("I will not ～" or "I will try not to ～") described here, followed by the quotative particle と. The sentence says "I try not to imagine the scene where he ..." You can rephrase it as 想像しないようにしようと(思って), although it's a bit long. The rule for using まい is found here (I don't know if we can call this "mai-form").

This まい is indeed literary (as opposed to colloquial), and tends to appear in written documents. But I feel it's not particularly formal. I haven't read this novel, but in a typical novel, there is nothing unnatural when you see まい in descriptive texts.