1)
C or assembler
besided mnemonic, what is the difference between

IE = 0x89;// C
and
mov IE,#089h;assembler

both require you fully understand what the IE sfr does and here is the crux:
C programmers assume that they do not need to know about the hardware (and, indeed, on PCs they usually don't)
Assembler programmers know that they need to know the hardware.

So the questions is not "should you program in C or assembler" but should you program with the attitude of C or assembler. The answer to that should be self-evident.

If you do not fully understand the hardware you will be lost in either language.

I have seen assembler 'programmers' that could not get their stuff to work blame the language for their shortcomings when the reason for the problem was a lack of understanding the hardware. In one case I got sacked from a consulting job because I pointed out the SFR errors and made the darn thing come alive instead of saying "I agree, switching to C will solve all your problems". The 'programmers' including the manager, had been bombarding management for months with requests to start over in C and spend 1 year+ to code it again.

If you can not get it to work using the language you are using, changing the language is going to make it lesss likely that you succeeed since on top of your lack of understanding the hardware, you will work with a language you do not understand either.

Erik