1. Steeping is probably better described as aging - it means letting your liquid sit around for a while before you vape it. For places like MBV that use lower-end flavor extracts (I believe they're pretty heavy on Flavor West), what this does is allow any alcohol/chemical aftertastes to wear off. While fruity vapes are generally less susceptible to these nasty side flavors than dessert/bakery flavors, if you don't like your liquid when you try it fresh, take the cap off and let it sit in a cool, dark place for a few days, putting the cap on and shaking it up once a day to evenly distribute the flavor throughout the liquid. Complex flavors may take anywhere from a couple weeks to a month to fully develop (more important with custom mixers like MBV than those who mix in bulk), but your green apple should be good to go in a few days if it's not ready as soon as you get it.
2. PG is propylene glycol - it's thinner, and users generally believe it carries flavor better, though it also adds a sharpness to the flavor, contributing to "throat hit." VG is vegetable glycerin, it's thicker and creates bigger clouds and smoother (but some believe muted or bland) flavor.
3. It's hard to define "don't vape that much," especially since how you'll vape depends a lot more on the tank you'll be using than the battery (Mini Volt) you attach that tank to. When I started, I was vaping about 2-3 ml/day on an OG Kanger Protank - now I vape about 1/8 as much nicotine but 5 times as much juice (maybe 10 ml/day). Others vape as much as 30 ml...
