Looks like there are pockets on the sides. Would those hold a glasses on one side and the pens on the other side?
To fit for the 2 differents place methods, the pockets are in different open direction. So the best is to put in the pocket on the right when in use.
Can it be used for an ipad mini or ipad air?
Absolutely, use it for an iPad or other tablets and kindles without problems.
Can the stuffing inside be replaced?
Yes. There is a hidden zipper on the bottom to access the filled area. You can take it out to replace or easy for wash.
Is it durable and actually useful for reading?
If there are wrinkles caused by the long-time transport, you can flat it by hand and hang out to fluff it back up. It can be change the angle by adjust the filled for better fitting.