Hello ZakM!
Yes, that’s my guessing too, 95% of it.
But, just like Maeda said, not all the cards are on the table yet. There are things that may not be predicted because we may lack of essential information.
A single chapter may change a lot of things, we know Maeda/Key.
Time travel, or something along those lines, is probable (it is just a gut feeling I have because of those -maybe misleading- subliminal hints and other things similar to the ones you’ve posted).
Also, “time” can be easily related to “Fate”. And this is a story about changing Fate among other things, according to the anime trailers.
But, what about the comet? We have no clue yet, it may be related with the time-lapse.
BTW, something that caught my eye from the beginning is the fact that they don’t use the words “time-travel”, they use “time-lapse” instead.
The comet will return, always.
I’ve got another gut feeling that the comet may be related to her little sister. But I doubt it.
Also, did you noticed how her hair shines when Ayumi uses her power? Seems similar to the person standing on the surface of the water in the OP.
“I will return”…
All of this is pure speculation, although many of this is quite plausible. It is a good cliffhanger to keep watching and enjoying until the very last episode.