The answer must be exact integers (you'd never sort 3.5 integers): e.g. something like this "for n=5 and smaller alg A is faster, for n=6 they are the same but for n=7 and larger alg B is faster" (a better way to write it would be "for n<6 A is faster, for n>6 B is faster, for n=6 they are the same"). Of course you might have some situations/problems when A and B will never run the same.
In general, you would get significant partial credit for good approximations (the better approximation, the more partial credit).
Similarly to the previous question, you can work here with integers (even though microseconds can be fractioned, unlike the input length). Namely, you can round your answers to the nearest millisecond. While it is useful - and actually not very difficult (hint: binary search) - to develop a skill for estimating in your mind the functions like (lg n), sqrt(n), with a good degree of precision, you can use calculators for that.
Yes, it will be acceptable for the credit, but it may often get in the way for both you and the grader. You should try to develop the style (feel free to use the book's one for example) which will help you prevent the technicalities from obscuring the core of the matter
It does not matter, as long as you state it explicitely.
In general, whenever you use an assumption, state it explicitely.