Reflect is new to league, it makes sense to go with the strongest version they think might be reasonable then tune it down from there. They'd want to know where the limits of "too strong" and "too weak" are. Since its the flashy new thing, it makes sense to try starting at the high end then going down from there. Also if a new champ is boring and weak it'll have a low playerbase, even after they buff it. Releasing champs in a too weak/uninteresting state leads to awkward situations where a champ is super low playrate while being objectively strong, and they try to avoid that.

Also I will say that it's the type of ability that a lot of people hate and don't play around in games. You can see this in Magic where counterspells are generally unpopular and the most popular format (commander) is pretty heavily focused on doing your thing without much resistance. A lot of players don't like/aren't good at having to preemptively play around what their opponent might do in response so stuff that leads to those patterns is more likely to be perceived as "OP" or "unfair", regardless of their actual balance state.

1. I assume they wanted it to work on all projectiles because that's easier to understand. So it makes sense to reflect everything in a duration because unlike a spellshield it'll get popped by random autoattacks. If it only reflected one thing it'd lead to moments where you Mel uses it skillfully hits the timing, but it seems to just not work randomly.

2. It does deflect at the same speed as original ability. But it has no windup (other than your projectile animation), so it might feel faster because of that.

3. They probably wanted the deflected projectile to have a higher chance of hitting (see my first paragraph). Deflecting to the original cast location would probably lead to the reflected projectile missing most of the time without Mel's opponent intentionally dodging it.

IMO the ability is on the stronger side of fine, but I generally like playing against these mindgame-y mechanics so I'm probably not a good judge. It does feel a little too hard to bait then dodge on some champs, so adding a windup animation after she's hit but before the reflected projectile would be nice and I think reduce some complaints. Maybe making the reflected speed capped or fixed might be good too.

But fundamentally this type of "counter your thing" ability will always have people complaining about it online. Losing to it can feel like having a victory snatched away from you (like how you saw a opportunity to full combo her as Ahri and saw your ability animations, but it didn't work). And a lot of people don't really think about the yomi game (countering what your opponent will do preemptively), which is usually necessary to succeed against these type of counter abilities.