@kado,
I suspect it is item 2 which is your main problem. I don't have a Nook so I can't really help you get to the bottom of that. Have you tried the Nook subforum? Surely, there must be many people there with the right experience.
I can help you eliminate problems with items 1 and 3. If you attach a TXT file saved in UTF-8 encoding which contains all the Turkish chars you want to check. I can create an epub with Calibre which will work on my Sonys and post a screencap for you to check. If it looks OK, that will confirm that Liberation is OK and that Calibre will put the @font-faces somewhere that an ereader can use them. The only thing then missing is the magic Nook url locations.
There is another way you can eliminate problem 2 altogether and that is to 'fully embed' the Liberation font files inside each epub. I think the perfect Turkish epubs you referred to were using that method. Calibre is not a great deal of help with this method so your task is a bit more manual but not difficult. I recently wrote a post outlining how to do this for Sony PRST1 users when we thought this was the only way we were going to get custom fonts to work on our shiny new readers (since then the magic PRST1 url location has been discovered so full embedding is no longer required for T1 owners). I am fairly confident this will work for you (as long as we confirm a good Turkish font).
Here's the link. It is referencing Gentium fonts to read in Vietnamese but much of the info is general. If you have trouble with the details feel free to ask.
If you do post the Turkish text I will also create and post a fully embedded Liberation epub for you so you've got a sample to examine closely.
In the meantime, I wrote another post a few months ago giving a
general overview of 3 different ways to customise fonts. It is not specific to any ereader brand but it may provide some background for you.