That would be a very good idea. Trying to suggest improvements to something when you don't even know how it currently works is a rather silly idea.
You are mistaken. Why did you think this?
The purpose of the difficulty adjustment is to maintain a ratio of 2016 blocks per fortnight. If blocks are generated too fast there are likely to be some issues due to the time it takes to propagate new blocks through the network. If they aren't generated fast enough, then transactions that require confirmations take longer to receive those confirmations. 2016 blocks per fortnight was chosen as a reasonable compromise and difficulty adjusts to maintain this. If total network hashpower were to drop (miners turn off their rigs due to reduced profitability) then the difficulty would decrease (which it has done in the past).
Then don't tell them that. Let them figure that out for themselves after they already understand the usefulness.
Bitcoin won't be deleting old blocks from the blockchain. They are necessary for the trust-less design. Some nodes may be able to trim out the spent transactions leaving only transaction hashes, but miners and full nodes will most likely always need the full blockchain.
9 GB really isn't much to secure the most significant development in electronic currency. I have more than 9 GB of family photos on my computer. I have more than 9 GB of RAM in my computer (meaning theoretically the entire blockchain should fit in RAM). I just don't understand why people are so concerned about 9 GB of data. Would it sound better if we said that it was only 0.009 Terabytes? How about if we just call it 0.000009 Petabytes?
It appears that there is more about bitcoin that you don't understand than just what a merkle root is. Heck, there's more about bitcoin that I don't understand. For example, can you explain to me the communication protocol that is used to synchronize the blockchain between two clients that have differing blockchains? Do clients request blocks/transactions from other clients or do they just sit around and wait for the blocks/transactions to be pushed? Bitcoin is a rather complex system, and to assume that you fully understand it is to set yourself up for confusion and false assumptions.
As for the merkle root:
The merkle root is the top hash in a merkle tree ( http://en.wikipedia.org/wiki/Merkle_tree ).