I went through the same process in 2006-7 so I can answer some of your questions based on my own experience
1) Does it mean that we will undergo another med examination and police check when the time comes to process visas? So applicants usually undergo 2 medicals in total?
Answer: Yes. There will be another medical and you will need new police clearance when your parents reach the head of the queue. Your information is in fact very useful because we were uncertain whether the first medical was still required. Your parents' experience seems to confirm that it is still a requirement
2) Have we actually been placed in a queue or not yet? I always thought that the Letter of Acknowledgement was a confirmation that the application has been place in a queue waiting for processing.
Answer: No. You are not actually in the queue until you receive a letter headed "Notice of allocation of queue date" or something similar. Until then you are in a queue to get into the real queue. The queue date will be the date on which you satisfy immigration that you fulfill all of the conditions for the award of the visa. Mainly these are the balance of family test, assurance of support, police clearance certificates and passing the medicals. Usually passing the medicals is the final condition to be fulfilled so the queue date will be the date on which DIAC receives satisfactory medical reports.
3) If it happens that we are not yet in a queue and our place will be allocated after medical examination and police check, then what Queue date we will get? Will it be the current date (say 01.01.2013) or the date when we actually lodged our application (10.09.2010)?
Answer: See the answer to 2). If you get satisfactory medical reports on (say) Jan 15, 2013, that will be your queue date.
In my case my wife and I passed the medicals in May 2007 but we were not actually told what our queue date was until March 2009. After a bit of trouble I managed to confirm that our queue date is in fact May 31 2007. Based on this experience you may have to wait a while before you find out officially what your queue date is but you can probably assume that it is within a month or so of taking the medicals.
With our queue date of May 31 2007 it now looks as if we will reach the head of the queue in mid-2014 - about seven years from the queue date or nearly eight from the first application. I think that waiting times have become slightly less since we applied. In the meantime you can get some of the benefits of having the visa right from the date of the letter of acknowledgement. If you are from the UK you are covered by the Reciprocal Health Care Agreement so you can get Medicare cards which can save you an awful lot of money.