Answer from cs61c-ck (Allen Lee 16583554) for Question 1
I guess the problem comes in when data can be represented as instructions.  Since everything is just 1's and 0's to the computer, it can't tell the difference between a good instruction and a bad instruction.  A person can conceivably reverse-engineer a program, add a few instructions that tell the OS to delete itself or something, and recompile.  Maybe a technique could be some heavy-duty write-protection but that in itself is also just written in data.