Sparkling Watermelon's time travel is consistent as of Ep08. Regardless, Sparkling Watermelon's time travel is simply, what we call in creative writing, a "plot device", it isn't _the_ theme. So, even if there are inconsistencies later, it doesn't really matter.
I think that's the thing that many people miss, how was time travel used. plot device or the central theme?
One more thing, the "hardcore genre fans" you are talking about, as far as time travel shows are concerned, are not necessarily "hardcore genre fans". A true time travel fan will always find a way to explain time travel without resorting to tearing "apart any story that fails to do so"; or using excuses like "writer error" or "lame writer" or whatever.
Unfortunately, in the K-drama land, there are very few us. And so far, I've been there to explain the craziness that most people were confused about, like in Alice and Grid, to mention two.
As far as time travel facts are concerned, we haven't proven anything. So, when it comes to fiction, any time travel fiction is acceptable. The problem is, most people think linearly and cannot grasp time travel fully. Hence, they make statements such as "that's not how time travel works". Again, Alice and Grid, are good examples, and I've explained those two.
Back to topic: Of course, time travel isn't new. Just that, when it comes to pop culture, Back To The Future became the icon of time travel so it is always referenced a lot. However, it is, in my opinion, the worst example because it is too linear. But we cannot blame it because it was meant to appeal to as many audiences as possible. If they made it complex, people wouldn't appreciate it.
And that's the point. Shows like Alice and Grid (again just two examples) are too complex, people dismiss it because they can't understand it. If it was like Back To The Future, it would have appealed easily.
^_^
