Thanks for all the wonderful feedback! I appreciate getting it, and I'm glad that you enjoyed your time with the game overall.
To answer your question as best I can, it is possible to play in a way that allows you to reach Good Friend status with almost every one of the "significant" characters. Basically if you interact with them and are able to gain acquaintance/friend/etc. statuses, then you can become Good Friends with them.
However, you have to reach Good Friend status with them prior to having them disappear or switching to them as your player character, because once either of those things has happened, they are effectively "lost" for the remainder of the game as far as being able to continue their quests or get to know them better. As long as you reached Good Friend status with them though, you'll get to see their extra little ending scene.
The one exception to the above is Madeline, as she is something of a special case. You can play as her if you've "died" in the game enough times, but no other characters can really meet her or interact with her in the normal ways like the remainder of the playable / significant NPC cast.
Mag isn't one you can befriend directly or play as. She appears mostly as a part of Wiz's story. It would have been interesting seeing things from her POV more though.
Digsby on the other hand, after you assist him with the crates, leave the area where he's usually found and come back to it again. He won't be at his shop stand, but he's elsewhere in that same area instead. Easy to miss.
Fun fact: Digsby is actually one where you can fail to progress his quest properly. If you chose to not help him with his delivery, you won't be able to become good friends with him.