From a strict linguistic taxonomy, and Austroasiatic are separate families, as distinct as English and Arabic. No mainstream linguist claims that Old Chinese descends from Khmer or vice versa.
A "Khmer-speaking Qin Empire" would likely have seen a fusion of Northern Chinese fortification and Southeast Asian temple-mountain aesthetics. The Terracotta Army
Epang Palace would look less like a Chinese courtyard and more like a "Mountain Temple" (Giri-style), with soaring prangs (towers) representing Mount Meru. 3. Religious and Philosophical Synthesis