Delete address and private key should do the job.

To backup, you go to import/export and click export unencrypted. Select Bitcoin-Qt as the format and copy everything. Save it into a textfile and into  somewhere safe. If you're scared, just copy it to a piece of paper and verify 6 times you copied it correctly*. Then, go to recieve money then click the "actions" beside the address you want to remove. Click archive address. Then go to archive, select that address and click delete keys.

Do this at your own risk. I doubt if it would help but you can try.
* A single letter wrong can result in the loss of Bitcoin.

You won't lose any of your Bitcoins as long as you export your private key and keep it at the safe place. I however, highly doubt that blockchain.info is rebroadcasting it. If you looked at the relayed by IP, it isn't blockchain.info who relayed it. Even if you remove your private key, if the person doesn't stop rebroadcasting your transaction, your transaction will stay there until its confirmed.

Unless they control 30%++ of the network, their own rules won't really affect Bitcoin much. It would be a bad move to include only high fees. They can easily accept all the fees and have much more profit than accepting only high fees.

Again, why would you want to carve it into a stone. Realistically speaking, that isn't cheap and if you wanna buy a specialized engraving device, it is only one use. It may last long but so do paper wallets and even hardware wallets. If you are afraid of forgetting your password, you're better off making several backups of a paper wallet or just use m of n multi sig wallets as I said. In case of fire, I doubt a carved stone would survive with all the letters clearly visible anyway. Technically, if you keep it safe, no one can steal it.

This wouldn't work if anyone is constantly rebroadcasting the transaction. Different client has different behavior, for blockchain.info, they stop broadcasting after 24 hours and the transaction will be dropped out of the mempool, it is never cancelled.

Most malware includes also a on screen viewer. The attacker can use it to view the password on your screen since the password has to be plain to be able to be pasted into the client itself. Furthermore, the attacker can easily hijack the clipboard which is fairly easy.

Coinbase, xapo etc don't allow you to control your own private key. If they gets shut down or hacked, you would have no way to recover your coins. [Not implying that blockchain.info is better but wallets like blockchain.info are safer etc. greenaddress.it.] Blockchain.info's fee can be specified under the custom send and you can set it to anything you want.

Trading of accounts is highly volatile just like any other trading activities. The price is dependent on the demand and the number of signature campaigns. You have the risk of getting a poor quality account that has little value for the post.

Possibly not, just add this in the terms and conditions. [Mining is resource intensive, I will not be responsible for any damage done to your computer. In no event shall we be held liable for any damage to your computer.] If they agree, they can't sue you since they know that you are doing cryptomining and it is resource intensive.

You can't respend a transaction that has proporgated well over the network. If the transaction is well proporgated and every miner and node have the transaction in their mempool, the nodes will all reject the second transaction and refuse to broadcast it. After a while, the entire network will drop your transaction from the mempool and the network will "forget" your transaction. You can then spend the transaction again with a fee. For blockchain.info, it would be around 24 hours. It defers for every client since many of them continuously broadcast your transaction even when it is dropped out of the mempool.

I tried sending 0.16BTC yesterday and it was confirmed serveral hours later. It had a medium-high priority due to the coin age. You transaction has a priority of 5,921,875, (3,000,000*370)/192. The minimum priority required for a high priority is around 28,800,000. I had mine above that priority yesterday but it wasn't confirmed fast enough due to the network load. There was a stress test going on yesterday which resulted in a spike of unconfirmed transaction. The inclusion 50kb free transaction size is by the ranking of the priority. Your's may be abit lower. Include a fee next time.

Some alt coins have algorithms which reduces or stop the profitability of GPU and possibly doesn't allow other ASICs to work. Although these coins would be profitable, they may have botnet miners already on them thus having high difficulty. In the first place, OP's 5 years old computer would probably have been under various loads and the performance would have been reduced significantly.

Blockchain.info unlike other online wallets allow you to control your private keys by yourself. They will send you a backup file and you can use that backup file to send your Bitcoins to another wallet. Same goes to greenaddress.it but they take a different approach. I still won't recommend anyone to use blockchain.info nevertheless.

You are only contributing to the network if you open port 8333 to allow relaying of transactions and blocks. Otherwise, it will just be downloading blocks from peer which does nothing

I believe the Core is sending some packets to the peers to get the blocks downloaded. You are only contributing to the network when you have >1 incoming connections.

Transaction are instant but not confirmation. Before one confirmation, there is still a risk of double spending even though it is relatively small. The reason why exchanges requires 3 confirmation is to have more assurance that a doublespend would have less chances of happening since the more the transaction, the harder it is to reverse (Not for 51% attack). If the doublespend is successful, the attacker could probably use the coins to crash any market thus tarnishing the reputation of the exchange. For everyday transaction, it is okay to accept 0 conf transactions if it's small.

Internet connection have little impact in the relaying of transaction. If you are going to broadcast a 10kb transaction [Never gonna happen, you would probably only need less than 1 second if you have the slowest internet in the world .

I consider it to be quite safe. I have found a reddit post which discusses about the security of BIP38. Even if someone does invent a scrypt ASIC to bruteforce your wallets, if you have a long character, lets say 11 characters. If the password doesn't contain dictionary words and is alphanumeric, it would be considered as relatively secure the longer the better of course. Afterall, to be able to crack it, they need to get the string of BIP38 encrypted private key which can be quite of a challenge.
It would be extremely unwise to point them to a online wallet if they are looking into Bitcoins. Numerous vulnerabilities have occurred with blockchain.info even though they allow the users to have control over their private key. Coinbase don't even allow the user to control the private key unless you opt for coinbase vault. Even so, they may have the potential of knowing your private key. You can easily point them to a SPV client which offers pretty much the same features with much more assurance.

Go to %appdata%. You will see a file called wallet.dat. That file contains the private key. You won't be able to read it using text editors however. You can either use blockchain.info to use the online wallet by going to https://blockchain.info/wallet/import-wallet or use pywallet to extract the private key before importing into electrum or multibit.I don't personally recommend blockchain.info after so much security glitches but you can still give them a try, they are still okay for a quick spending.

Keyscramblers only work if it is a simple keylogger and not a malware with sophisticated features. A attacker can potentially with elevated rights [by using means of exploit] bypass your key scramblers. It will not work if you don't encrypt your password though. It will also not protect you from physical keyloggers that intercepts your keyboard connection to the computer. The most effective protection is to not click on suspicious links or run/download any suspicious programs. Also, make sure you update your computer frequently.

It is important to note that many of them are pump and dump scheme. The price may just crash when you try to cash out. It is quite risky. If they are really profitable for a long term, the old gpu/cpu miners would be mining on the coin thus rising the difficulty.