(Seems like we're not getting anything more specific, so I'll format my comments as an answer for ease of future reference.)
The electronic edition of the Kokugo Dajiten has it like this:
接頭語「ま」の下に促音の挿入された形,
So the standard view isn't that it was makoto > maQ-; but rather a simple ma-, to which -Q- was later tacked on. And indeed, of the words you've cited, most of them have non-geminated attestation in historical documents; though that, in itself, isn't decisive, since gemination wasn't always written down consistently. Still, I think the standard analysis is more likely true than not.
Why? If you consider some prefixes ending with -ki/-ku that create gemination, you'll find these are often on’yomi, i.e. Chinese loans, like 悪- aku- in 悪口 akkō, akku or 石 seki- in 石鹸 sekken. But these trace straightforwardly to Middle Chinese readings ending in consonants, such as *ʔak and *dźak in this example (compare the modern Cantonese readings: ok, sek). We know these loans retained their final consonants even in Japanese for a good while. It's natural that a syllable-final consonant would coalesce into a geminated one
What that has to do with anything? Well, ma- is a native Japanese prefix, seen in the 8th-century Man'yōshū in expressions like ma-kanasimi "truly sad", or 旅とへど真旅になりぬ "I said I'd set out on a [little] journey, but—lo and behold!—now it has become a real [long] one." Linguists have many reasons to believe that Old Japanese had no gemination, so ma- was a perfectly cromulent prefix by itself, predating the introduction of geminated consonants into the language. In fact, it's very likely that makoto itself is just ma+koto 真事/真言, true things/true words (again as per Daijiten).
Since we had a native Japanese ma- rather than a Chinese mak-, there seems to be no reason for it to trigger gemination. True, the Japanese onbin changes also triggered gemination, like torite → totte; but not with [k], since the onbin form of kakite resulted in kaite instead. Then whence the -Q- in maQ-? Consider that both historically and synchronically, Q is added to create emphasis:
- matashi, mataku (historical form) → mattaku (modern form)
- → まっっったく (tongue-in-cheek superemphasis in manga or the Internet, etc.)
- yohodo → yoppodo (both forms still coexist)
- mina → minna (also works with nasals)
Consider also that there are several other emphatic prefixes with a -Q- (取っ, 引っ, 打っ, 押っ as in 押っ死ぬ、押っ放り出す…), and also the use of Q in interjections like "こわっ!"
Given the meaning of 真 as "true, truly, for real", it makes sense for it to receive emphatic gemination, which in time lexicalized into forms like 真っ黒, etc. We can predict from this that, if a word has a 真 prefix but didn't get an emphatic っ fossilized into its lexical entry, it could still get a っ for extra discursive emphasis; and in fact, forms like まっこと or 真ん夏 are easy enough to find online.
