Wow -- lots of questions there.

The GMATPrep score is pretty close. It should be within 30 points either way.

To get a 51 on Quant you need to get very few questions wrong... whereas if you score a 48 you can miss lots of questions, 10 or 12 or so. It is actually a big difference.

You can go faster by practicing. There are types of questions that you will recognize as you study. You will see that these questions are repeated in disguised form...but you can understand what the question is doing and you will be faster.

For sentence correction you need to understand a few things, first is that much of what is written in each sentence is intentional clutter. Learn to recognize what can make a sentence wrong and what is harmless. Modifiers that are not underlined and not touching are generally harmless sense they are isolated from the part of the sentence that is underlined.

Also understand how much of sentence correction is a test of logic. Does the modifier logically modify what it is touching? Is this a logical comparison? Does this meaning make sense logically?

You should have plenty of time per day and plenty of days to improve so that you are closer to your goals.