It will not help trying to slim down your code with those things. You want too much functionality in a basic Nano board.
If you have used digitalWrite() just once, then it will take not extra memory to use it more. However, to put the parameters on the stack requires a few extra bytes and direct register access does not. Those few bytes here and there will not help your sketch.
You have already many basic functions in your sketch. When you add new code that use those functions, then you can do a lot with little extra Flash memory usage. With 72% Flash usage, there enough to add more things.
I think you have to organize your code and debug output better. Put code and variables in *.ino files and have a #define DEBUG_... per *.ino file and be able to turn on and off whole sections of the code.
In the past I used a ATmega8. I had to disable for example the OneWire library and return a fake temperature to test the VirtualWire library.