To whom does "va'Anachnu" apply?
Radak: It is we, who fear Hashem, and recognize that He is the Master of everything. He will revive us, and we will bless Him in life.
Maharsha (Pesachim 64a): It refers to the third and last group to offer Korban Pesach. 1 They are called lazy (65a), so they are not blessed "Yosef Hashem Aleichem...", "Beruchim Atem..." (verse 14-15). Even so, we will bless Hashem!
Malbim: It is we who are alive. In addition to praising Him, we bless Him via our deeds, which cause increased influence and Brachah. We bless Him, that He is the source of influence and Brachah.
Pesachim 5:7: The first two groups had many people; sometimes the Leviyim sang Hallel three times before they finished offering their Pesachim. The third group was always small; the Leviyim never reached "Ahavti..."
What is "me'Atah v'Ad Olam"?
Radak: From now until the world to come. When we die at the allotted time, our Nefashos will not perish; they will always bless Hashem.
To whom does it say "Hallelukah"?
Radak: You who fear Him, and you are still alive - praise Him in your life, and He will sustain your souls after the body dies.
