It didn't improve on its existing playstyle. Prankster sableye has an effective niche, but magic bounce boosting already belongs to two other pokemon and is questionably effective.
Yes, and neither of those Bouncers can take a hit for shit, nor do they have a decent way to either heal or gain Pseudo Bulk with burns (no Espeon ever runs Morning Sun). Hell, Espeon only works on BP, where other mons have to do the boosting for it.
Mega Sableye has better typing and (almost certainly) better bulk than either of the two, not to mention (compared to Mega Absol) it can make use of its base form for priority recovery, spreading Burns on Physical Attackers, and/or priority boosting (patches up Special bulk if using CM).
And Foul Play wouldn't be totally useless. If Sableye chose to run it, it gives it a Pseudo-Mixed Attacking Option, since enemy burns don't drop the attack power. Mandibuzz can use STAB Foul Play already (though it's full defensive), but unlike Sableye, it has no way to present its own offensive presence, and Sableye can spread burns in its base form to ease switching in, only further aided by the Defense boost.
On the contrary, I think Sableye could actually pull games similar to some Gyarados sets, playing its base form until it needs to Mega evolve, be it setting up or shutting down Pokemon so its teammates can punch holes. Unlike Gyarados, Sableye is a bit harder to shut down in a sweep, since it wouldn't care about Intimidate, Magic Bounce avoids Status from Pranksters like Gyarados and Zard-X have to have dealt with first, and it has two spammable STAB options.
Might not shoot up to S-Rank, but it gives Sableye's opponents new options to be prepared for, and does allow some maybe gimmicky but some effective ways to toy with the two niches.