(1) I'm 5'9" so I almost always have to get denim hemmed and I've never had a denim specialist do it. The "keep the original hem" is something most alterations places will do now b/c so many jeans in the last few years were distressed and it looks weird to have distressed jeans with a nice clean hem. However, it adds a little bulk to the bottom of your leg and can make cuffing funky, so take that as you will. Real chainstitching pre-wash allegedly gives you "better" railroad tracks (distinctive patterned fading on the hem) but how much that bothers you obviously depends.
(2) I'm neither tall nor thin and I think I look OK in Ervell. Maybe not ideal, but OK.