In general I think it's correct to say double negative and question are used to save face but I'm not sure if this sentence is double negative.
The expression 来てくれるん is not a negative sentence and also the expression
～じゃない？ does use the negative form but is not really a negative sentence either and is more like a question.
For example, something similar in English may be
Isn't it cold?
Japanese translation would be
～んじゃない？ is used to say something with some assumptions and also kind of asking for agreement from the listener just like the example above.
Some examples are:
(You see dark clouds) Don't you think it's going to rain?
B: レストランに 忘れたんじゃない？
(You saw your friend use her/his wallet at the restaurant) Maybe you left it at the restaurant?
～かな... is used to make it not sound like pushy when stating your opinion. I think it's because it sounds like you are stating your opinion as if you are asking it to yourself or talking to yourself. It's like
hmm I think.