I would still allow verbal spells. I think it's important to acknowledge WHY a wizard can't cast a spell in a grapple.
The idea is that it disrupts the concentration needed to cast a spell. So I'm allowing cantrips with no check, since they are written to not require much effort in 5e.
Re disarms, an opp att doesn't make sense to me, nor does allowing a DEX save to resist once the hit has occurred - DEX already factors into how hard it is to hit someone - but nor do I want it to be particularly easy to disarm. Hm, maybe the DC should be the greater of 5 or half damage, rather than 10 - I used 10 as that's the base DC for Concentration checks, but maybe that's too high. Putting it at 5 further discourages mooks from attempting a disarm, and favours sneak-attacking Rogues & big-damage types, which makes sense to me.