How did it come about historically that っ preceding a sound
would geminate it? Is it really a little つ or are they just near
homomorphs? This question should be broken into two different
questions: (For those who don't know the term: gemination simply
means doubling of sounds, usually consonants. It's easy to get
the sense once you see it comes from the same Latin root as
Gemini). The answer for the first question is simpler, and was
already given: in historical kana usage, consonant gemination
was originally represented either by a full-size つ or by another
consonant that got elided (usually that would be く, as in 学校,
which was written ガくカウ in old spelling, or き as in 石鹸,
originally: セキケン). The small-tsu spelling is recent. It was used
sometimes even in the 19th century to indicate that the sound
got elided, if not earlier, but small kana letters became a
standard only following the 1946 spelling reform. The answer for
the second question is more tricky. I don't have any historical
linguistics books that I c