You could, but if you think you need to do so in your code, I'd suggest rethinking/rewriting your program. If you manage to leave pins in a state that you're not sure about, this means you're not really in control of what you're doing.
It activates the internal pullup resistor.
Yes. Write 0xff to PORTC, PORTD etc. Again, the question is why you'd need this. The only conceivable scenario would be that the delay of 62ns (assuming an ATMega328p running at 16MHz) between toggling individual pins through direct port manipulation is too long of a delay. If that's the case, I'm very interested in hearing about what kind of project you're working on.
There is by reading the port direction register (assuming an AVR architecture, others may refer to it differently). But, there's no reason for doing so. Your code sets the mode, so you obviously know what it is.