It's quite an original time-travel story. Sybill Trelawney has nightmares which lead to the development of snitches that cause the 1998!Tonks to be transferred to the body of 1990!Tonks. Tonks introduces changes that lead to Harry being put in the care of Augusta Longbottom.
I like how this story isn't centred around Harry, but concentrates on minor characters like Sybill Trelawney, Nymphadora Tonks, Charlie Weasley, the Weasley Twins, the Defence against the Dark Arts professor (an original character; I like him), a few Slytherin characters (all original; it's nice to see Slytherin student being ordinary students).
But your original character Orin is a problem. He just annoys me. An Oren from 2015 goes back to 1990---that's 25 years compared to Tonks 8 years---and it's completely irrelevant for most scenes involving Oren!!! Why is Oren a time traveller??? He has 25 years of advanced knowledge and he's not using it!!!
Oren saves the diadem horcrux---but more because it's part of Britain's cultural heritage, than the fact it is Voldemort's horcrux. He hates that Dumbledore supporters destroyed the diadem, but is silent on the issue of Voldemort using it as a horcrux. He blames Dumbledore for the death of the Unicorns, but it is Voldemort possessing Quirrel that actually does the killing. He hates Dumbledore for reading people's minds. Yet, when he confronts Snape about reading peoples mind, he's satisfied when Snape states he isn't interested in teenage drama--- he just doesn't bother assigning the same reasoning to Dumbledore!
Sometimes Oren appears like he'll be the first in line at the Death Eater recruitment station. Other times he appears like a middle aged man having had a mid-life crisis and now having gotten the chance (literally!) to have a second childhood. He has no problem with having a young teen-aged girl in his lap. He has an irrational hatred for Dumbledore and his supporters. And he's only a passive supporter of Voldemort. I like him as a slice-of-life Slytherin character, but I dislike him as a time traveller, because by virtue of his action (or inaction) he's isn't one.
Author's response
Your impression of Oren is pretty much as intended. He is not a hero or adventurer. His values are not mainstream, although he thinks they are. He isn't fully rational himself, although he believes the world is inherently rational and talks accordingly.
If the question is why did the snitch spell pick him, I'd point out that Acamar wasn't very experienced with that sort of intelligent magic and we don't _exactly_ what he told it to do. As the author, this is the MacGuffin for getting multiple time travelers working at cross-purposes.
As to the teenage girl, on the one hand that was more experience with girls than he had ever had in his previous life. He can pass for professional, but is socially very awkward. In the story he is going back and forth between exploiting his sophistication, covering up for his sophistication, and covering up for (or not thinking about) the fact that he's not all that sophisticated in the first place.
On the other hand, at the end of the day, ethics about that sort of thing are not exactly his strong suit.
I once went to a talk by Lev Grossman about his _Magicians_ books, where someone asked him why his characters were so irresponsible, and he replied that none of the characters were exactly "the cream of the ethical crop" (or maybe it was "swimming in the deep end of the ethical pool"; I forget). And that is exactly the case in _Let's Try That Again, Shall We?_, too -- there are characters (e.g. Arthur Weasley) who are clearly and reliably _good_, and who will go out and do the right things every time. This is mostly not their story.
This seems like a good time to reiterate that the opinions of the characters are not necessarily those of the author. :)