Thinking on it, with Double-Rate Eph, Umi opens with the Blind spell anyway, and Eph almost surely misses one of his subsequent two attacks, right? Then Umi should be able to take it from there if I am not mistaken.
Fair point. Blinded Eph is best off with a Killer Lance; even at halved crit rates, 25% Hit and ~20% post-nerf Crit get him ~10% chance to kill Umi per attack sequence. (In between Cute Pose and Siren's Call, he needs a hit and a crit.) So:
Umi uses Blind (84%), it must hit.
Eph attacks and doubles, a crit will kill.
Umi uses Cute Pose, defenses up.
Eph attacks and doubles, hit+crit will kill (or two pre-buff hits and two post-buff hits, etc, I estimate a 16% chance Blind Ephraim has killed Umi by now.)
Umi uses Siren's Call.
Umi uses Elec Strike, 61% chance it hits. If it misses, Eph counters and then attacks; I'm definitely not going to try to break that down, particularly since Ephraim's damage numbers would be different
yet again, but my offhand guess is that Eph's looking fairly good if he makes it that far.
That said, overall I expect the scenario still favors Umi, but that is under the assumption that the Blind spell a) is available (Blind Strike is clearly superior in a 1v1 vaccuum and they're mutually exclusive), b) ignores FE evade (FE evade does cover spells, and FE8 Stone in particular is a status that checks Evade) and c) that halving crit rate and factoring crits into the average are appropriate for FE8. Offhand, I am inclined to favor all three of those options, but going the other way on either of the first two would put the match clearly in Ephraim's camp, and I assume reversing on the third would also work in his favor, which means I'm leery of leaning Umi. Flip side, assigning DL opponents AS based on their speed compared to their average and FE8 enemy averages would... force me to decide whether turn based speed buffs should be allowed to affect effective AS.
Bottom line is, I remain more comfortable betting on Ephraim.