I think the double spaces things depends, honestly on the size and type of font. If it is teeny tiny type in an awkward font, I think there should be two.

I think that putting in quotes at the start of chapters can work, but you need to be careful.

First person is fine, as long as its believeable, and look at Agatha Christie and The Murder of Roger Ackroyd. Actually after the book was published many mystery writers used this ploy until, as John Dickson Carr said, it got to the point where every time you picked up a book with a first-person narrative you started to worry that it was going to "be that man again" as he put it.