1)
What is the meaning of "Bnei Avadecha Yishkonu"?
1.
Radak: They will dwell forever in their land, after they will return in the days of the Ge'ulah. They and their children will never be exiled from it again.
2.
Malbim: Even though the entire existence will cease, Bnei Tzadikim will dwell forever without perishing. This is in a generation of Tzadikim who are a part of Hashem; they are attributed to him, like His years. Due to them, He makes changes in His world.
2)
Why does it say "Lefanecha Yikon"?
1.
Radak: They will serve Hashem all days.
2.
Malbim: Tzadikim will stand in front of You in the world of Neshamos.
