I think one example that might be a richer source of "the origins of interrogatives" is Chinese, from Old Chinese (Classical texts) to Middle Chinese (Literary Chinese texts) to modern topolects of Mandarin, Cantonese etc.
The reason is because the interrogatives are much less stable than in Indo-European languages, and have changed many times. There is also a very large synchronic diversity across Sinitic topolects, with the e.g. Cantonese interrogative system very different from modern standard Mandarin or from colloquial northern Mandarin.
Across the history of Chinese, the "what/which + " is the most common interrogative used for time, place, manner as well as non-personal nominal phrases, and has been generalised to personal nominal phrases in many topolects. In Classical Chinese, these would have been the velar series in Middle Chinese phonology (according to Pulleyblank, with */ɣ/). This was most commonly written with 何, but variants abound.
The "who" for personal nominal phrases was separate in the Classical language, starting with a palatal affricate in Middle Chinese (Pulleyblank uses */dʑ/), and most commonly written 誰. Its reflex remains for Mandarin, but many southern topolects now use the "which" equivalent + personal nominal phrase.
A third family in Classical Chinese starting with a glottal stop in Middle Chinese, typified by 安, is hypothesised to be mergers of the prepostional coverb 於 (is, at) with various elements. I'm only aware of the Min group preserving this reflex.
In the history of Middle Chinese, one of the transitions from Early to Late Middle Chinese (through the Tang dynasty) is the dual layer of Classical vs more vernacular interrogatives in text from this time. The Classical 何 structure for the "what/which + " had in competition the more vernacular 什摩 (note the characters!), which caused a few changes in syntax for the interrogative adverb of manner (the how). The origin of this new "sh-" family seems to be 是物, literally this thing in Classical Chinese, but [apparently] taking on interrogative force in late Middle Chinese. This of course led straight to Mandarin 什麼, Min Dong 什乇, Min Nan 啥物 etc.
There are lots more issues to go through, but I'll just list a few different things for the reader's own research:
- the which/what distinction across (most?) modern topolects
- the emergence of the t- class in Min
- the emergence of the m- class in Cantonese/Yue, Hakka, and Mandarin
- wh-in situ vs wh-movement across the topolects
- the emergence of interrogative numerals (幾何 in Classical Chinese, 多少 vs 幾 in Mandarin)
- the origin of the families of Classical Chinese interrogatives and their relation to the Sino-Tibetan family at large (for example, Tibetan uses as one of its interrogative pronouns ག་རེ, Wylie transliteration: ga-re. What evidence is there that this is connected to 何? Can we reconstruct a proto-Sino-Tibetan form?).
- What trends are there for the use of different characters along different periods or maybe different areas for each of the Classical families?