Re: Ukemi and Koshinage
Preface: I learned ukemi by being thrown - many, many times by yondan and sandan judoka. However - initial instruction prior to this was... landing 1/2 way between side and back, "bottom" leg a little flexed at both hip and knee, "top" leg landing foot first, consciously keeping the knees apart to prevent thighs from causing compression to the sensitive "naughty bits". Vigorously slap mat with arm closest to mat JUST prior to torso hitting mat.
That said... practicing with experts who will throw you "correctly" will make it easier to take the ukemi as they will direct you towards the mat in a way that you will be able to handle. As your skill in ukemi improves, experts will throw you away and you will be left to your own devices to protect yourself as you land and recover from being thrown.
Sorry this probably wasn't much help...