Coin days is not flawed. If you have 100 coins and let them age for 10 days, you have ten times as much stake as someone who has 10 coins and let them age for 10 days. Compare apple to apples, not oranges.
1. Then people can just keep reusing stake.
2. Cryptocurrencies needs to be decentralized. You can't identify who one "miner" is.
3. See 2.
2. Couldn't you design the coin so they are identified by their public key in the blockchain?
Also, your example doesn't make sense to me. The days are an unnecessary addition. Throw the day measurement out entirely and you get the same results. 100 coins > 10 coins. Age doesn't matter.