FixedFloat is an exchange. However, if you first swap BTC for XMR, move the XMR to your own wallet, change your IP, and then redeposit the XMR to swap it back, you achieve privacy. However, I'd guess it's more expensive than just coinjoining the BTC.

This depends on the specific situation, but in general yes, I believe Monero in practical use is more likely to result in more privacy and anonymity compared to coinjoins and similar solutions.

My suggestion is that if you want to understand how to actually achieve privacy, security and anonymity, learn from the people whose life literally depends on it.