Mae Change

Wait until any L&D fodder becomes available; she would greatly benefit from it. In the meantime, Darting Blow could work

My Mae has Darting Blow with a +spd nature and she wrecks. Mine has Blarblade+ tho. Can't say she consistently doubles everything, but the majority of units do get doubled. You even have Blarowl to boost your spd even more, so I think it's fine if you give her Darting Blow 3.

If I were you I would keep fury and certainly DO NOT GIVE HER LND.
Giving her LND with the +Res upgrade is like a failed attempt at patching her.
I think you should run with what you started.
Fury is actually really nice as it boosts her SPD but also her RES. You can make her a very good mage tank aswell as a good damage dealer.

The thing I would recommand however :
As she is an Owl user, it means her gameplay is dependant a lot on her allies for positionning and using the buffs to the fullest. It's okay if you don't always have 3 allies around her as it's not always possible but 1-2 should be easy.
My suggestion is to give to thoses allies some spurs. That way you can move around easy and position easily. Also the advantage of spurs is that they can stack. It means you can actually give one of her ally spur SPD as a C slot and Spur SPD Seal. This would allow Mae to gain 6-8 SPD (depending on the level of the seal).
And that is not even considering the owl buffs. With 2 allies next to her she will get a bonus 4 spd.

Her spd right now is 34, with the owl bonus it would be 38 and with the spurs from your ally 46, you can also add the ally support effect and reach 48 spd.

I don't really think you should replace Fury with either darting blow or LND.
A big part of the gameplay with Owl tome is to bait and kill the ennemy. LnD will undermine that as she will be more fragile and Darting blow is a pure player phase skill.

Also I would consider also giving spurs or buffs to the 2nd ally. If you really wanna go over 48 spd you can always go with hone spd but it might be overkill. I think you could go with a Spur RES and Spur RES Seal to get her a nice 50 RES (36 base, 4 owl, 8 spur, 2 ally support). It's nice to note that with this RES she will be able to tank any Red and blue mages (including Reinhardt) and also do pretty good against green ones.

You could also use an ally with spur ATK for a more damaging approach of course instead of the RES spur.

Just so you know, I'm also running a Mae with similar setup. I run her in a mage comp alongside close counter Boey, Arvis and Julia.
Julia can give her +6 SPD (she doesn't have the Seal maxed yet) and Arvis +6 ATK (He also need to upgrade the seal).
She doubles almost everything and I don't need darting blow at all.