For Futaro - this link gives a good description of what is going on with the on'yumi/kun'yumi issue. http://www.tofugu.com/2010/03/23/the-types-of-kanji-in-japanese-onyomi-vs-kunyomi/
It's common practice that in compounds you'll be using onyomi.
It's not that difficult. The fact that there's a hiragana 'ki' after it tells you that it should be 'oki'. This has nothing to do with this grammar though...
How can I know how must I read the kanji, if in on yomi or kunyomi. For example: in nﾂｰ 5446 is: koko ni "TE" ki ppanachi or koko ni "OKU" ki ppanachi? It is very difficult for people who donﾂｴt know japanese very well.
It is a colloqual form of 放し, meaning doing something and leave it at that.It has a lot more emphasis than ...たままにする.
This is a rare case of 一級 grammar that I hear in every day conversation. I learnt this when I was still a beginner. I think it has a similar meaning to "〜たまま".