So far invented such a scheme. The whole amount to be exchanged for any local currency. Surplus since last investment to be converted back to BTC. Remaining - initial investment - to be spent on daily needs.
What is the point of exchanging every Satoshi into Fiat if the profit is converted back to Bitcoin? Then simply sell the initial investment for Fiat and keep the rest. Your scheme is too complicated in my opinion.
First: Does this way of turnover looks reasonable at all?
It is reasonable, but only as long as you make a profit.
Second: How to make sure I use "clean" BTC from source of exchange?
Bitcoin is non fungible. If you are afraid of using a platform because they may start treating you like a criminal, you better simply stay away from it and find an alternative. Preferably, a platform that does not see their customers as criminals.