Can you explain how do I prove that:
1) I sent my coins to a Mintpal deposit
2) I withdrew the coisn to a Mintpal deposit
If mintpal doesn't exist?
If you deposited and withdrew BTC from Mintpal, those transactions are permanently recorded on the blockchain. It doesn't matter what transactions took place on Mintpal's internal database.
You sent BTC from Address A (which you can prove control of). You received a higher amount of BTC at Address B (which you can prove control of). The difference is your BTC profit, but what matters for taxes is the USD profit. Each trade on Mintpal is taxable, and the net of those gains determines your taxes due.
The fact that Mintpal no longer exists just makes it harder to disprove the records you provide for your Mintpal trades. That's why any good faith effort to pay taxes on the gains made between Address A and Address B is probably good enough. You made 4 BTC profit, which had a USD equivalent when you bought back BTC. That's your taxable gains.
About unreported income... if you receive an income here in exchange of working in signature campaigns, as soon as you give them one address, they could link it to an account, the look for every address ever that you used with your account and they could see how much money total you've made with your account, and then accuse you of not having reported that income when you sell a part or all of it. So im not sure how this could work out.
How can they definitively link addresses together? Personally, I am extremely careful about linking wallet addresses together, even if it means paying higher fees to consolidate outputs.