1)
What is ha'Misderonah?
1.
Rashi citing Targum Yonasan: It is an Achsadra (a room without four walls). It came out from the Aliyah.
2.
Radak: It is where people (coming to the king) sit, Sedarim Sedarim (arranged, e.g. in rows). 1
2)
What is the meaning of "Ba'ado"?
1.
Radak: On [Eglon], like "va'Yisgor Hashem Ba'ado" (Bereishis 7:16). This was lest people of the house enter before Ehud leaves.
3)
How did he lock it?
1.
Rashi: He locked it with a key. 1
QUESTIONS ON RASHI
4)
Rashi writes that he locked it with a key. How did he have the key?