Valve doesn’t use super high detailed models or fancy cinematics, they just use in-game models with some nice renders in SFM. In other words, there’s no super highly detailed model of Alyx out there available to us (that I know of anyway). I think the most they do, if anything, is add a little more detail to the mouth and use better flexes.
You should be fine using the Episode 2 model of Alyx if you consider that trailer highly detailed and HD. It should only need a few minor tweaks if you wish to even do that much.
As far as obtaining HL2 models for model editing, people usually just decompile them them from the game unless there are raw files available. Some of the model data was released in the source sdk folder for people to reference and use but I don’t know where it’s moved since the steampipe update.
Cinematic mod is always an option as stated above, but that’s not really Alyx anymore. That’s basically an overly detailed model of Adriana Lima with ridiculously large textures and a ridiculously high poly count.