Those kinds of covers are almost always flashes forward in time, especially when the author doesn't have the chapters to show it. Rarely ever random. Similar idea to making a tiny extra chapter. In this series there isn't any doubt they like each other romantically so there's not much to worry about.
Yeah you've got the gist of it. In truth, what the author can actually depict in the manga is dependent on the editor(s) and when a series gets axed, the cover art is usually all you've got left to draw "what you intend." A lot of yuri comedy series can never get to the dating/marriage/etc. phase because the magazine doesn't want it, let alone the fact most of them don't even allow for romance unless it's left with plausible deniability.
The author herself has talked about how she was really happy that the Gundam Witch anime was able to show their protagonists married with wedding rings—if she was allowed to take OshiYuri that far, she definitely would have. Alas...
You're absolutely supposed to interpret the cover as a flash-forward on what the author wanted to draw.
Damn, this series was so fun and different. I wish it had more time to cook. We live in the darkest timeline, where this doesn't make it to 20 while so many lesser titles are freely chugging along to 40...or 400.