1)
What is the "Devir"?
1.
Rashi: It is the Amah Traksin (a wall one Amah thick that divides between the Heichal and the Kodesh ha'Kodoshim). Also refer to 6:5:2:1*.
2)
Why does it say "Hechin"?
1.
Radak citing Yuma 52a-b: He saw with Ru'ach ha'Kodesh that in the end, the Bayis will be destroyed. He prepared a place underneath to bury the Aron shortly beforehand; Melech Yoshiyah buried it there.
2.
Gra: He prepared that there will be place inside for the Aron.
3)
What is the meaning of "Lesiten"?
1.
Rashi: It is like Lases (to put). Radak - it is Makor (an infinitive). Sometimes the suffix Nun is added, like "b'Avdan Moladti" (Esther 8:6), "Heshivosa me'Charon Apecha" (Tehilim 85:4).