With.a block explorer, you can trace where bitcoins came from, so it might be possible to see the transactions going into the addresses that you think are in your old wallets and match them up with records of your sales. That is going to be tedious and it might not work very well.
The best option is to recover your old wallets so that they can show you all of the transactions that you care about.
Another option is to just estimate the number of bitcoins you received and their values using the records of your sales.
No. Coinbase will send you bitcoins from a different address every time. It is more likely that it is an old blockchain.info wallet. However, if you did receive bitcoins in a Coinbase account, they will have records of your transactions.
My advice (and it might be really bad advice) is to just do the best you can, even if involves making guesses or estimates.