What should I do if I have wronged someone verbally and cannot reach them to apologize, and will I be forgiven if my tongue slips with a word and I then seek forgiveness?

If you have wronged someone from whom you cannot seek forgiveness, then increase your prayers for their forgiveness (istighfar) and remember them with good where you had remembered them with the opposite. Likewise, if seeking their forgiveness would cause mischief, then istighfar for them is sufficient. Every sin committed by a person, whether with their heart, tongue, or limbs, if they then repent with sincere repentance (tawbah nasuha), Allah will accept their repentance and forgive them, and they will become as if they had never sinned.