These all can not be done in C.

I don't know about can not, but it won't be trivial; again you will need a good understanding of both assembler and 'C' - plus a good 8051-targetted compiler and a good understanding of its 8051-specific extensions!

- variables located on fixed addresses
- code mixed with constant data

These can be achieved with the 8051-specific extensions in Keil C51; I guess other 8051-specific would incorporate similar extensions.

- functions with more than one return value

You'd have to use tricks like global data, pointers, or returning a structure or union.

- functions with more than one entry point

Ooo-errrr

- using 9-bit numbers (ACC + CY)

That might be possible in C51