I bought BTCs from bitcoin.com.au around 20 hours ago and it is still showing 0 confirmations in my wallet. I checked on the blockchain.info (I am only a newbie) and to me it looked as if the bitcoin address from which they sent had unconfirmed deposits into that account. Could it be the reason for my transaction not being confirmed?
Yes. And that transaction comes from a long chain of unconfirmed parent transactions. All paid around 222 to 300 sat/B fee. I think that's high enough to confirm those 10 or so transactions within half a day.
What happens if the transactions doesn't go through for a few days and get lost? does the coin go back to the seller/sender OR he can double spend by associating the higher network Fee?
Both can happen.
