Does blocking “Remote Content” in the Mail App create make-shift Private Browsing e-mail?

It'll provide you with similar protections as long as you don't click on the Load Remote Content button. If you want to protect yourself using this feature, you need to ensure it is enabled on all devices your mail account is configured on, including in the iCloud.com Mail preferences if you ever log in to read your mail in a web browser.

The Duck service is an interesting one. On one hand it's a nice touch to simply remove the trackers from the email before it gets put in the inbox, however there are two problems I foresee here. First, formatting can go astray, as some of the trackers are cleverly embedded into core graphics of emails sent our by companies. Secondly, you can't send from the @duck.com address, which means some organisations will not accept that it is a valid email, especially if you need to contact their support team via email over sensitive matters.

My opinion is that the Apples new mail privacy features announced under iCloud+ tackle this in a far better manner, where the images are simply pulled down once when the email is received and from that moment each time you access the email the image is served from Apple's servers, thus obfuscating any tracking to any one individual. That's not to say that there isn't a place for Duck's service, and it certainly will be useful for many users, and I for one welcome the competition.