Dark Excalibur Merric...

I have a 5* +Def,-Atk and don't really know what to do with that...

But in your case, I would stick with the +atk, -spd cus you'll probably want to keep his defenses and you can always patch up his speed with the sacred seal. As for his passives, if you have the options later on, I would consider Triangle Adept with either Quick Riposte (for mage retaliation and helps with -spd) or Axebreaker. With TA and his decent defenses, you won't need Lancebreaker at all. Otherwise, Death Blow would work out well since most axe users are slow and bulky and having extra attack never hurts. As for the C slot, Threaten Res is always good but Threaten Atk (although more harder to get) helps with his DC tanking.

go with the +spd -def merric.

35 speed is pretty good and life and death brings him into 40 spd which is already very optimal and he appreciates the +5 attack. Speed is very important in the meta, so don't focus on attack too much because you can do more when you double.

He performs better than sonya as a glass cannon because of the extra 1 point in speed.

Sonya is still better at damage output and tanking because of higher attack and res but she also doesn't have enough speed to double a lot of high tier meta threats without a breaker or darting blow. This is where merric outclasses her because he can run life and death+desperation and is a better glass cannon while sonya can serve you a bit more as a tank. it also brings him on par with a spd boon sonya with darting blow except with 2 less attack.

That's a cool upgrade for Merric indeed but I'd still stick with my Sonya since this game is about playing your favorite heroes more than anything else really :p

I'd only consider Dark Excalibur on Merric if you can afford CC + QR3 fodder. Basically using him as a pseudo green dragon, pure enemy phase bait unit.