Here "presence" stands for the people serving as witnesses. AT: "with my fellow countrymen as my witnesses" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This phrase shows that Ephron was part of that group of people. It does not mean that he was their leader.
# I give it to you to bury your dead
"I give it to you. Bury your dead"
# your dead
The nominal adjective "dead" can be stated as a verb or simply as "wife." AT: "your wife who has died" or "your wife" (See: [[rc://en/ta/man/translate/figs-nominaladj]])