1)
After teaching that they should build houses, why does it add to plant gardens?
1.
Malbim: Gardens show permanence [of residence] more than houses. It says "Lo Yivnu v'Acher Yeshev" (Yeshayah 65:22; rather, they themselves will live in the house, and not an heir or stranger), and the verse adds a bigger Chidush, "Lo Yit'u v'Acher Yochal."