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1. I currently have one wallet address that I'm aware of. Am I correct to assume that this wallet address I now see on coinbase.com is the same wallet that's an app on my phone? I guess another way of wording this question would be: Does accessing my wallet via coinbase.com while on a PC vs accessing my coinbase wallet on my iphone mean I have two wallets, or are they both the same wallet?
You don't have a "wallet address" at coinbase because you don't have a wallet. You have an account at coinbase with a deposit address. I suggest that you avoid using the coinbase wallet on your phone because coinbase has two apps. Both are generally referred to as wallets, but only one is actually a wallet, and that ambiguity can lead to the confusion demonstrated by your question.
If you are going to use coinbase, install their app, but not their wallet.
You can't mine Bitcoin with a PC, but you can mine other coins and get paid in bitcoins. That is what NiceHash does. In order to decide whether or not to mine coins, you need to know your revenue (the amount you are paid by NiceHash) and your costs (the cost of your electricity and equipment). I think NiceHash is good for starting out, but I don't know what to do with the problems you have. Check the Bitcoin Forum > Alternate cryptocurrencies > Mining (Altcoins) forums here for NiceHash info as well as info about mining altcoins.