To whom did he write "Hashem Nesancha Chohen"?
Rashi: To Tzefanyah.
Shemayah was a Rasha. Why would he care who is Kohen [Gadol]?
Radak: Even though [totally] spiritual Resha'im] denied Hashem's Nevu'ah, they agreed that Hashem is the first cause [of everything].
The other Kohanim make the Kohen Gadol the richest. Perhaps he arranged (or hoped) that Tzefanyah would share some of his wealth with him.
Why did he mention "Yehoyada"?
Radak: He was the Kohen Gadol, and the greatest of his generation. And so Tzefanyah was Kohen Gadol, for it says "in place of Yehoyada."
What are "Pekidim"?
Rashi: They are appointees.
Who will be "Pekidim"?
Radak: Tzefanyah and the other Kohanim.
Why does it say "Beis Hashem"?
Radak: This is like b'Veis Hashem. The same applies to "ha'Nimtza Beis Hashem" (Melachim II, 12:11) and similar verses.
What is "Meshuga"?
Radak (here and Melachim II, 9:11): They used to call a Navi Meshuga for detriment, for at the time of Nevu'ah he acts like a lunatic - "also he removed his clothes" (Shmuel I, 19:24). He loses his senses and powers of his soul. Evil Yisraelim, who followed Ba'al, disgracefully called Nevi'ei Hashem 'Meshuga'im.'
What is "ha'Mahpeches"?