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How much beer/whisky would one have to consume to be chayav m’doaraisa?
Also why biblically is there a prohibition on liquid chometz if it’s in the form of a liquid and not leavened bread? We know by other areas of kashrut once you break it down or change the form then on a biblical level it’s no longer the same entity. We find this concept by rennet/gelatin. I’m not saying any of this is allowed as far as Halacha l’masseh.
The Rambam writes (MT Chametz u Matza 1:1)
Anyone who intentionally eats an olive's size of chametz on Pesach [...] is liable for kareit [...] [The above applies] equally to one who eats chametz and one who converts it into a liquid and drinks it.
So the question becomes how much whiskey contains an olive's worth of barley. According to this site, it takes roughly one kilo of barley to produce one bottle of 70cl of whisky. If you believe an olive is 27cm3 (Wikipedia), which for barley converts into 16.2 grams (according to this site), then an olive's worth of barley is 1.13cl of whiskey (16.2 grams * 70 cl of whiskey / 1000 grams of barley).
As to your second question, of why there is a prohibition on liquid chametz, the Gemara in Chulin 120a says
Reish Lakish said: One is liable even for drinking leavened bread. The verse states: “For all who eat leavened bread from the first day until the seventh day, that soul shall be cut off from Israel” (Exodus 12:15). The term “soul” is interpreted homiletically to include in the prohibition one who drinks leavened bread.
Now all is to be chayav m’doaraisa. As is well known, one if forbidden to consume chametz on Pesach in minute quantities, as the Radbaz (quoted at the end here) nicely explains
Therefore, I rely upon that which the Rabbis taught that chametz symbolizes the “evil inclination,” the “se’or she-be-issa.” Therefore, a person should utterly banish it from his midst and search for it in all of the inner chambers of his consciousness, as even the smallest amount is not nullified. And this is true and correct.