1/ Which covenant was confirmed, "with the many" for one week? (Daniel 9:27)
This refers to Jesus new and everlasting covenant for the forgiveness of sins through his spirit. This new covenant was sealed by his death and resurrection. The destruction foretold in Daniel is the Roman siege of Jerusalem which took place in 70AD. https://en.wikipedia.org/wiki/Siege_of_Jerusalem_(70_CE)
Here 3 days before Passover when the majority of Jewish people in the region had descended on Jerusalem to celebrate the feast day - Rome sieged the city - burn the temple to the ground (As foretold in Mat 24:2-14 https://www.biblegateway.com/passage/?search=Matthew+24%3A2-14&version=NIV) and murdered or captured the vast majority if Jewish people alive at the time. They turned Jerusalem into a pagan town called Aelia Capitolina. The abomination of desolation.
Jesus new covenant was essentially a way to "rapture" to heaven the righteous Jewish people that would be killed in the coming destruction
2/Who does the many (people) refer to, and why did God extend the period?
The covenant was with the children of Israel but it was later also extended to the Gentiles through his disciples. All those who under went water baptism for symbolic repentance and then received the holy spirit.
Galatians 3:7-9, 14-18, 29
3/ Is the covenant in force today?
Yes. Whilst primarily it was targeted to the Jewish people in order for them to escape the coming destruction from Pagan Rome - Jesus advised it was "an eternal" covenant. And one which was later extended outside of the jewish people to the nations of the world though the mission he assigned to his 12 disciples after his death.