Possible to defend myself from plagiarism in draft thesis?
According to my own understanding of what a "draft" means, it sounds like you're getting a raw deal here.  A "draft" is an informal document that is in the process of being changed and improved.  Unless "submitting a thesis draft" is something formal in your program, to me this sounds like accusing someone of cheating for looking up the answers to a practice test.  I also view the fact that you included the citations as a key one in your defense: that makes it quite clear that you are not in the middle of a larger plan to steal others' work; you just turned in a shoddy draft.

You do however sound very naive about what plagiarism means and how academic writing works.  You don't write an academic document by taking others' words, modifying them slightly, and then planning later to modify them enough to make it not plagiarism.  At no point in this are you actually doing the independent thinking and writing that's asked of you.  If you want to indicate in a draft that you intend to include standard background material, you could include a section that is almost blank with "[Standard background to be filled in here as from SourceX]".  The less generous way to look at your situation is "How did Mr. Ske manage to make it all the way to a master's thesis without understanding basic academic norms?  If he hasn't understood these things by now, it's too late."

The other issue is that, sorry to say, the rest of your description of your academic experience sounds extremely (virtually entirely) negative.  You have a bad relationship with your advisor -- very bad; he raises his voice at you when you ask questions, and he directly mocked you; both of these are really unacceptable -- you have a coadvisor who didn't read any of your previous drafts and gave you papers that you found "irrelevant" (which they might be, or you might be mistaken about it; either way is bad).  Finally, someone who should be helping you out instead reported you to the Dean's office.  This is really not good!

To answer your question: accepting everything you've said as an accurate account of the events, I personally find it unlikely that you will be expelled for this.  However I am much less optimistic that you can move forward from this to successfully defend your thesis and complete your degree.  The behavior exhibited by your advisor and coadvisor could well be gearing up not to accept your thesis.  Someone who accuses you of plagiarism is surely pretty close to being done with you whether the accusation is affirmed or not.  I'm sorry to say it, but I think you should be looking for exit strategies and considering other options.  It is certainly worth a shot to go to your advisor and say "Look, this has been a bumpy road and I acknowledge that.  I've been working on my degree for XX years, have spent YY months working on my thesis, and I would really like to finish it if at all possible, even if it is not perfect or what we originally hoped for.  I would really appreciate your help in showing me what I can do to make that happen.  I know that the primary responsibility is mine, and I'm willing to put in lots of hard work between now and ZZ, but so far I haven't met expectations, so getting more specific direction will really help me through this."  Almost all advisors want their students to finish rather than not, so that is a good shot at salvaging a bad situation.

Good luck.

Added: Here is another site on the internet containing some discussion on the issue of plagiarism on a thesis draft.  Note that the case in question concerns missing references, which is not exactly the same as yours and to me seems more serious.  The overall consensus is that initiating formal plagiarism proceedings for this is so far over the top that there must be something else going on beneath it, which is my thought as well.  However they point out that a ruling will depend on the precise plagiarism rules at your university, which (i) of course we can't know and (ii) of course it is your responsibility to know.  But again, reading between the lines here, there is something profoundly wrong with your relationship with your coadvisor, so trying to fix that directly has got to be a good idea.  The fact that it will not nullify the formal plagiarism inquiry is just an (unsurprising) formality: that's just saying that once something has been started, you can't unstart it, you can only end it.  The only person who is accusing you of anything is your coadvisor, so if you can work things out with him, the inquiry will almost certainly work out in your favor.  Hinting that you did this because of his bad advising seems like the worst possible strategy to me.

Finally, my talk of "exit strategies" does not mean that you should simply give up: in fact, as I immediately followed, you should have a frank discussion with both of these faculty to see whether the thesis and degree can be salvaged and if so how.  But to be realistic, you sound like you are in an unusually bad situation: if you are at a university where master's students repeatedly get turned in by their advisors for what they write in their thesis drafts: yes, consider the prospect of going elsewhere if you can.  Starting over is not the end of the world, and sometimes it is the best way to move forward.