Since you asked direct questions I will give direct answers.
In your score range (44Q), the last question probably could have improved your score by 10 overall points if you answered it correctly. By the last question of a section, the CAT has zeroed in on your score and even the last 2-3 questions probably won't move your overall score much.
Will you be on equal footing with those applying to top 5 shools? Well, this is going to sound harsh, but no. The average GMAT at these places is around 710 and heading up. The types of people that gain admission to these places have excellent grades, excellent work experience and excellent recommendations and great stories to tell about their leadership and involvement. If you have each of the above qualifications, but a GMAT score 30 points under the average, your chance of admission will be lower than their average admit rate, probably by at least half.
The more I research these schools the more convinced I am that the only people that can gain admission with GMAT scores below the average are under-represented minorities and people with truly unique and relevant work and personal experiences (senator's kid? Donald Trump's daughter? fulbright scholar? charity missionary in Sudan?). Certainly, we have all heard stories of people with 650 GMAT gaining admission to Harvard, but by and large it just doesn't happen.
The good news is that you seem to have a good chance for improvement. You said that generally you do better on the Q, and your history with the SAT certainly shows some promise. If you are serious about the top 5 schools, I think you should defintely consider taking the test again. If you are flexible and are willing to work with top 10-15 schools, and if you are very strong in other areas of the application, you may not have to. For top 5 (really the 'ultra-elite' top 7) schools, you need to be be strong in every area and exceptional in at least a few.
I know people are going to get their panties in a bunch over my comments so I will throw out some hard numbers for everyone to think about. Everyone can apply their CR and DS skills a little here. 200,000+ people will take the GMAT this year. 680 is the 89th percentile, which means there will be over 22,000 higher scores this year. The 7 ultra-elites (as defined here at GMATclub) will have total class sizes of approx 4100 students this year. Certainly some of those will be people with scores less than 680 as well. (Looking at admitted totals does not work because some of the same people will be admitted to all the same places; looking at class sizes is the only thing that makes sense). So, maybe 16-17% of those scoring over 680 will gain admission to the top 7; and certainly those with higher scores will have the best chance. To go one step further, there will be 16,000+ scores over 700 this year. There's still just those same 4100 total seats in the ultra-elites.
In talking with Adcoms I have more and more come to believe that while an excellent GMAT certainly is not enough to get you into a top school, a below average GMAT will go a very long way in keeping you out.