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On my kanji dictionary, below 七, there are several kunyomi among the common ones (nana, nanatsu), they are: na, nano, nanu; it's written that they are nouns themselves and it's the same for the other numbers (for example, fuu for 二, mi for 三, mu for 六, etc.; they're all nouns).
So, when does someone say na, nano, nanu as a noun alone for "seven" instead of shichi/nana? Did they do it in the past?
No, I think very few people will understand you if you say なぬ、な、or なの individually to mean "seven". Although they are listed as readings, they only make sense when the are in compounds. If you look up 七 or "seven" (as a word) in a dictionary, the only pronunciations that appear will be なな、しち and ひち, which is dialectical.
Although they are listed as readings of the kanji, they will only make sense in compounds. なの is used in 七日【なのか】, which means the "seventh day of the month. なぬ is obscure and also seen in 七日【なぬか】. I have never even heard it pronounced this way. な is not even listed in デジタル大辞泉 as a pronunciation. I recommend that you remember the pronunciation of words and kanji compounds rather than the pronunciations of a single Kanji.
Unless you use the kanji to name your child in a creative way (which is quite popular nowadays), you cannot. Only "nana" and "shichi" can be recognized as 7 alone.
I think you'd agree with me, if you are an English speaker, that you couldn't pronounce "raz" to mean rasp, even if you say so in raspberry; nor "bird" to mean board, in spite of cupboard; nor "sul" to mean sail, while topsail and studding-sail.
Although every dictionary has slightly different criteria listing kanji readings, what is safe to say is that most of them are variants only appearing in compounds. A kanji dictionary is sometimes awkward, obligated to list all possible "realizations" of the kanji (= fixed spelling). It may result in an unexpected outcome, such as 生 has tens or perhaps over 100 "isolatable" readings (non-isolatable ones are 熟字訓).
