First: Does this way of turnover looks reasonable at all?
Reasonable, as long as you enter and exit at the right time. It's faster to make a lot of money using this strategy compared to lump sum or DCA. But I don't have balls to flip my money, so I choose to invest with DCA strategy.
If you spot the market correctly then yes this method will work and be profitable, but what is the success rate of this method I will say it low. I know many will argue with me that having a very good knowledge in both technical analysis and fundamental analysis will make it a success but I put it that no matter how good you’re at this analysis the risk of it getting wrong is still high, bitcoin is not that predictable no matter how you use those analysis, take for example even successful traders who trade daily they will tell you that their losses are mostly more than the wins. So DCAing and Holding as you have said is the best way to go
Second: How to make sure I use "clean" BTC from source of exchange?
Make sure:
2. Whenever anyone want to send coins to you, ask them send them directly from centralized exchanges.
3. Never do P2P.
2. If the don’t have bitcoin to send from centralized exchanges then you can simply give them a custodial wallet address first to send the coin into and then you can actually transfer it yourself to an exchange, this reduces the risk of receiving tainted coins directly into your exchange
3. There is nothing bad about the P2P if only it is done on that exchange, the exchange will have not allowed the P2P with tainted coins on their platform