Depending on how serious you are about "getting government approved", you can do a cross chain analysis on yourself and provide necessary view keys. To prove you bought 10K of Monero, just show that your bank account went down 10K and your Monero account went up 10K (at the past exchange rate). To show you exchanged 5K Monero for 5K Bitcoin, show that 5K Monero was transferred from your wallet at the same time 5K was added to your Bitcoin wallet. It's extremely cumbersome and requires that you have access to all wallets/accounts and is extremely invasive on your privacy, but it can be done if you really want to. Personally, I'd agree about the tax attorney. Often there's a way of "legitimising" grandfathered funds, but expected to be taxed to the max. Alternately, there may be a way of doing crypto loans so that you get the money without actually cashing out.