Grammar notes 46

Another way to express obligation is with the て form of ーない, ーなくては(いけ|なら)ない.

と may be used at the end of a sentence to produce a differential, akin to when/if.

ーないで is used when the following sentence is a subordinate to the previous statement, but not an underlying cause. On the other hand ーなくて implies there is a reason, so that first statement is the reason of the following statement.

なくはない is a phrase, as such any closer look would be redundant, it would be equivalent to "you could that that, but you could also do this".