Seal Priority

depends, as you said you have some -Spd units so unless you have a unit that really needs distant defense i would go with speed

If one of your arena core units is someone that's meant to bait mages/archers like a rein/b!lyn counter, then DD will help them tank both. Otherwise, spd seal is just generally useful in all situations

DD3 is so so so good in arena. I'd recommend that first unless the extra 1-2 speed from upgrading that takes whatever unit you'd use it on over the hump and let them double and kill specific units you're struggling against.