They have always told me they come from Chinese.
All the sources say they come from Chinese.
Even you said "some theories".
Keep it to facts, buddy boyo
Some characters derived from chinese. But most of them came from bonji. Read the link that I have posted.
Not only that, the sequence of letters in the hiragana/katakana follow the similar sequence as siddham.
For example:
Japanese vowels: A, E, U, Ae, O
Siddham: A, Aa, E, Ee, U, Uu, Ri, Ae, Oi, O, Ou
Japanese consonants: ka, ki, ku, ke, ko
Siddham consonants: ka, kha, ga, gha
Etc.
Also, there are many grammatical similarities, specially in the rule of phrase, compounds and S-NP-V rule etc.
What I'm saying, Japanese is way more closer to curry language than western/european languages.
There are also many vocabulary overlaps with Pali, Sanskrit and Japanese.