Q1. Yes, OG questions are ordered by level of difficulty. OG questions normally have easy questions in the first part, medium in the middle part and hard questions in the final set (last 20-30 questions i guess). People say that OG questions are way easier for 2 reasons. Firstly, OG provides a higher percentage of easy and medium questions. You get very few hard questions in OG. So, people find it easier to solve OG questions with ease but they find it little difficult to solve in the actual exam. Secondly, OG has retired GMAT questions. True.. But the level of difficulty in the questions that come in the exam is growing as time pass by. This strategy employed by GMAT test makers makes the exam more and more competitive. Otherwise, you would start seeing more people getting more than 700 which would eventually be a cakewalk for everyone.

Q2. Yes, even in real exam you will be given an erasable sheet with marker, similar to what you get from MGMAT. Some people may find it suddenly difficult to use the erasable sheet with marker in the real exam. So, MGMAT provides this kit for those people who can get more familiar with real exam testing conditions.

Hope I've answered what you were looking for.