As you've noticed, the key sentence to deconstruct is the last one.
In either case, both the object and the creature or solid surface take 3d8 bludgeoning damage.
"In either case" refers to the case where the projectile is stopped by "stopping early if it impacts against a solid surface" versus the case where it is stopped by "the object strikes the target and stops moving".
Then the rest of the sentence is straightforward: "both [the object] and [the creature or solid surface] take 3d8 bludgeoning damage" - [the creature or solid surface] will refer to whichever stopped the projectile, and both that and the projectile itself will take 3d8 damage.
No, because the Spectator has to save against the Spell itself, and the Thorns are not a spell in and of themselves
The spell Hail of Thorns does the following:
Target: Self
...
The next time you hit a creature with a ranged weapon attack before the spell ends, this spell creates a rain of thorns that sprouts from your ranged weapon or ammunition. In addition to the normal effect of the attack, the target of the attack and each creature within 5 feet of it must make a Dexterity saving throw. A creature takes 1d10 piercing damage on a failed save, or half as much damage on a successful one.
In this case, the spell doesn't actually target the Spectator, it buffs you. The enemy being shot is not the target of the spell and is not saving against the spell, it is saving against something the spell has created: essentially a bomb made of thorns.
For example, if a Caster launched Fireball at a Spectator and the Spectator succeeded, that would count as saving against a spell. But if Fireball was cast on the unstable cave ceiling above the Spectator, the Spectator might still make a saving throw, but if it succeeded, it couldn't retarget the rocks.
For another example, if a Caster attempted to Polymorph the Spectator, the Spectator could attempt to retarget the Polymorph if it succeeded on the Saving Throw. If the Caster Polymorphed an Ally into a creature, and that ally-creature missed the Spectator with an attack, the Spectator couldn't retarget Polymorph.
In this case, Hail of Thorns could not be retargeted because the Spectator is not saving against a spell, it is saving against a bomb which was made by a spell but is not itself a spell.
Edit: A Tweet from Crawford supports the idea that Hail of Thorns targets the Self and not the Spectator:
A range of self means the caster is the target, as in shield, or the point of origin, as in thunderwave (PH, 202)
https://twitter.com/jeremyecrawford/status/606193562317766656
Best Answer
The PC Gets a Save
There is no effect line in 5e's magic spells, so when it says affects another creature its not calling to a mechanical keyword, but saying it shifts target. Spell Reflection is basically used to turn the party's spells against them, but a targeted PC should still get to make the normal save as if he/she was the original target of the spell.