Personally, Supergirl dying/almost dying has lost most if not all of its impact through repetition. ("Oh, this is where she gets beaten to death--again..."
I would be against an extended absence for the character, as we hardly got enough of her in this season!
I suppose one of the reasons I'm okay with a temporary death is because the "near death" has only happened twice, and was never done particularly well. We had the potential for it, with Reign. But the follow up episode, which should have been about everyone's emotional reaction to Supergirl's defeat, was completely botched.
And as great as "Red Dawn" was, the intensity of that fight did not seem to warrant the "near death" that resulted from it.
As for an extended absence for Melissa, there are SO many ways to explore that time. As I said, you could have one "anthology" style episode, where citizens of the city are remembering different times when Supergirl saved them. So Melissa could be there for that.
You could also have another "Midvale" flashback episode, as Alex and Eliza remember Kara.
Then you have an episode about crime ticking up, as people try to take advantage of Supergirl's absence.
I suppose the main reason I want the "death" isn't just to see Supergirl die, or anything like that. I want the EMOTIONAL impact from the rest of the characters, and the world at large, as they think their hero has fallen. Not just Alex, but Lena, James, Brainy, Nia, Eliza...people she's saved in the past, like that girl who was part of the Cult of Rao. It explores the impact Supergirl has had on both her loved ones, and the world. Not to mention the immediate reactions from the other heroes involved in the "Crisis". In the comic series, Supergirl was the first major casualty. And seeing the type of impact that has on Oliver, Barry, and even Clark, could provide some really wonderful dramatic scenes.
This type of emotional fallout is something we haven't gotten with either of Supergirl's previous two near-death experiences.