Has every person had an opportunity to accept or reject God's salvation?
Yes. The Holy Spirit is omnipresent and works on every human being. Every person who has ever lived has been given opportunity by the Holy Spirit to accept or reject God's salvation, just as hyssop was available everywhere so no Israelite could claim he had no chance.
Les teaches that hyssop grew everywhere in Goshen, comparable to ragweed, so no Israelite could say he had no chance to find hyssop and apply the blood. He likens hyssop to the work of the Holy Spirit. Because the Holy Spirit is omnipresent and works on every human being, Les is firmly convinced that every person who has ever lived has been given opportunity to accept or reject God's salvation. No one can ever say the Holy Spirit never worked in his life.