1. Thanks, sorry I do not understand まともじゃない. I do not even know how to parse it, except that I recognise じゃない, of course. I do not know what まとも is grammatically, or what it means.
2. Just to be clear, I thought 入らない was はいらない, but I find it hard to distinguish between 入る（はいる) and 入る（いる） except that I think I was once told that 入る（いる） is mostly used in set phrases, so unless it looks like a set phrase I assume はいる; is that right?
I thought it was intransitive and the subject was 手紙, as you said. Is it not semantically possible to use the potential form? Is it because the letter doesn't have the possiblity for volition or something?