Implied Question: This Pasuk was stated regarding the first exile (after the destruction of the first Beis Hamikdash). [How do we know that such a prohibition to go back to Eretz Yisrael applies even after the second exile?]

Answer: It is possible to answer that the Pasuk must also have been stringent that people should not go back to Eretz Yisrael (in this fashion) after the second exile.

Implied Question: [What secret is this referring to?] This is referring to the secret of arranging the calendar (making leap years, months, etc.), as the Gemara in Shabbos (75a) states "for it is your wisdom and understanding" (Devarim 4:6) refers to the secret of arranging the calendar

Implied Question: [What does this mean that the righteous will get up from the dead "in their clothes?"] This implies in the clothing that they were buried in, namely their burial shrouds.

Answer #2: The Yerushalmi says in the chapter of "ha'Nosei" that Rebbi said to his children that they should not spend much on burial shrouds, as the righteous will stand up (from the dead) in their clothes. This implies that they should not spend much on shrouds, but rather should spend the money on clothes when they are alive (as the righteous will wear the clothes they owned while alive when they rise from the dead).