Does the intention of benefiting others invalidate the deed, if the primary aim was not the reward, and is that considered showing off (riya')?

If you do good deeds seeking nearness to Allah Almighty, and desiring to benefit others and guide them, then you will be rewarded for that. This intention includes seeking recompense and reward from Allah Almighty, and this is not Riya’ (showing off), because Riya’ is when you perform an act intended for the sake of Allah, but you desire the praise and commendation of people through it. What you have mentioned does not fall within the meaning of Riya’.