Elincia +ATK -SPD

Of course she's still good, just means she'll quad less often. Though tbh, I think quadding is more of a fancy gimmick to proc luna, the +atk boon lets her be like a red cherche who can simply nuke people on the first 2 attacks with deathblow, especially if you use QP+moonbow

her with +Atk and Deathblow has teh exact same power as a Brave Sword +Atk Chrom using Deathblow, but her speed will still exceed his for avoiding more doubles and she's a lot more mobile than he is, with the advantage of being able to use unrestricted Hit & Run as well.

If you try and quad strike with her, you'll be using L&D anyways and you will actually still have a speed of 36, which is about where most other quad units end up at as they usually favor attack boons. Her reduced speed penalty gives her a lot more leeway in her bane stats as a result and quad striking will be just as viable as a +atk Hana in this case.

Any Elincia is insane. At worst, you're not abusing her best features. Merging them would give you +HP & Spd, which does wonders for both. +Atk ensure she has enough Atk to 2HKO most things, but the extra +1 Spd off a merge allows her to have 37 Spd with L&D2, a Spd Seal and 38 with L&D3 + Seal. Neutral Spd maintains her amazing Spd allows her to exceed 40 with said setup.

It's kind of up to choice, but I'd take +Res/-Atk really.

Depending on which IV you select I think your build will be very different.

If you choose the +ATK, I'd keep Death Blow and give her Hit & Run as her B skill to maximize the damage given by her initial double attack.

If you choose the -ATK IV, I'd choose L&D3 and Desperation to focus on Quadstriking opponents.

In my opinion, the Hit & Run build works great within a flier team or a team with a dancer because it will help you get out of range of enemy attacks during enemy phase while the quadstrike build is usually overkill and leaves you quite vulnerable.

I vote for the +ATK IV.