Nuendo has a 32-bit floating point engine.
There is absolutely no need and/or advantage in having a 64-bit audio engine, since summing is nothing but adding and substracting numbers. Critical processes (where cumulative errors can occur) however are always executed in double precision, 64-bt. Actualy, most processes (which is not the same as adding/mix engine) are performed in double precision.
ASIO is fixed point, and fixed point only since it has to communicate with Hardware. And since there is no Floating point hardware commecially available, there is no point in writing a floating point ASIO driver. As far as your integer-question, see following.
AFAIK, there is no such thing as a 32-bit fixed point file format.
See above answers.
Best regards
Fredo