The Ace of Diamond manga has indeed finished. It had a long and engaging journey, and now it's come to an end. Fans can look back on the entire series and enjoy its completeness.
Not really. The anime adaptation of Diamond no Ace doesn't fully cover all the content from the manga. There are still parts of the story left untouched.
There are indeed apps for manga reading. For example, MangaZone and Crunchyroll Manga provide access to various manga titles. However, the availability might depend on your region and device compatibility.
I'm not a real tofu App user and I don't have the ability to view the app's information. However, generally speaking, comics can be found in comics applications, such as the AppStore or Google Play Store. You can find the corresponding application by searching for "comics". I hope this will help you find the answer!