May one make a minyan on Shabbos afternoon only for Kriyas Hatorah, without davening Mincha?
Yes, a Minyan just for Kriyas Hatorah may be done for Mincha of Shabbos. The only change would be that a Chatzi Kaddish would be said right after the Kriyah (as opposed to when Davening Mincha – i.e., the Amidah right afterwards – when the Chatzi Kaddish is recited by the Chazan while the Sefer Torah is being returned to the Aron Kodesh).
(Of course, if a Minyan is being arranged just for Kriyah not in a Shul etc., one must make sure to follow the special guidelines that apply in such a case. See here.)
See here regarding a similar topic.