The reason that the product of the slopes of lines parallel to the x and y axis will not be -1 is this: any line parallel to the x axis will have a slope of 0 (rise 0 / run... infinity), and any line parallel to the y axis will have a slope that is undefined (rise infinity / run 0). Any time you're dividing by 0, the fraction is undefined. If we multiply the slopes, 0 * undefined = undefined.

I've never seen this rule directly tested on a GMAT question, but of course it's possible that they might test it. (The GMAT very rarely asks about situations in which a ratio is undefined). Almost always, when the GMAT is testing perpendicular lines on a coordinate plane, they'll give slopes such as 1/3 and -3; these are negative reciprocals whose product is -1.

No, you won't need to memorize the codes of the schools you want. There will be a drop-down menu.

You won't need to resend the scores - once you've sent them, the schools will keep them for up to 5 years (after which time the GMAT scores expire).

You will be given one pad of laminated scrap paper. If you run out, they'll bring a new one and take away the old one. I'd recommend getting a fresh pad during each break so you don't run out.

Good luck!