Can one take a Siddur off the shelf five minutes before Shabbos is over for Ma’ariv, or is it מכין משבת לחול?

No, unless you use it a little on Shabbos.

See also here:

What is the earliest one can Daven Maariv on Motzei Shabbos when it is the first night of Yom Tov such as Pesach this year?

שמירת שבת כהלכתה פרק כ”ח סע”ד והערה קע”ג.

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