I am not able to understand the difference between a bitcoin transaction block and a bitcoin as a coin.
Transactions are coins?
There is no difference. This is because there is no distinct data entity that is a bitcoin. The nearest thing to a coin is a part of a transaction record where a recipient's amount (an output amount) is recorded which has not subsequently been spent by appearing as an input in a later transaction. These unspent transaction outputs (UTXOs) are arguably the nearest thing to a coin.
Miners melt coins?
The idea of coins arises because of an analogy with fiat cash. You can regard part of the miner's job as effectively melting down the input coins and recasting the molten metal into some new output coins each with a different face-value. The total weight of metal remains the same.
If you want to spend money you don't give anyone coins, you just spread a rumour out that you'd like certain coins to be melted down and specify what size coins you'd like to be cast from the melt. For each new coin, you specify a new poem to be inscribed. You prove you are the author of the poems melted.
Coins don't exist!
Analogies are not real, they are just stories to try to help us understand something by referring to something different which we already know about. In the case of Bitcoin these analogies and most of the terms are really deeply misleading at times. Wallets never contain money. Keys don't open locks. Addresses don't identify places (or people). The blockchain is not a ledger. So don't get hung up about what a coin is. A coin is just a character in a fairy story. It isn't real. It is a starting point for understanding, but it won't take you far. After a very short while you will need to avoid thinking about coins, if you want to better understand what is happening.
in the initial days did the author(s) had to create lot of unnecessary transaction
All transactions are unnecessary!
(... for mining to occur, apart from the miner's own)
Blocks added to the bitcoin blockchain don't have to contain any transactions other than the so-called coinbase transaction where the miner creates a permitted amount of new money out of nothing. That is a transaction with no inputs, only an output. Such blocks are still created from time to time. However miners are financially rewarded for including other people's transactions in their blocks, so this is relatively rare.