What is the Chidush that "Al ha'Aretz" must be in the last line? They are the last two words!

Rashi: They must be the only words in the last line.

If "Al ha'Aretz" is at the end of the line, this is like an extra Parshah break. The Mezuzah should be Pasul!

Daf Al ha'Daf: Pri Megadim (Mishbetzos Zahav 32:26) left this difficult. Kol ha'Torah (55 p.20) says, Bach (YD 275) disqualifies only when one made an open Parshah closed, or vice-versa. However, Taz there challenged this. Perhaps regarding a Mezuzah, all agree to Bach, like we said above that if text that should be written normally was written like a Shirah (a part of the Torah in which parts of the lines are left blank), in a Sefer Torah it is Pasul, but in a Mezuzah it is Kosher. Teshuvas Rema (25) says that Tefilin and Mezuzos need not be written like they are in a Sefer Torah. Since they are not written to learn from them, space between Parshiyos to contemplate does not apply. There was a tradition about which Parshiyos are open, and which are closed. However, Magen Avraham (32:33) says that a space of nine letters in the middle of a Parshah disqualifies.

Daf Al ha'Daf: Rivash asked why we needed to say that there is a Pesul of writing like a Shirah in a Sefer Torah. In any case, the spaces are like a Parshah break! He answered that this is the proper form for a Shirah (so it is not considered a Parshah break). Perhaps since it is normal to finish [Kisvei ha'Kodesh] differently, a space [at the end] does not indicate a [Parshah] break. The Rema (275) says so about the opinion that we finish a Sefer Torah in the middle of a line.

Why do we want to maximize the distance between "Al ha'Aretz" and "ki'Ymei ha'Shamayim"?

Rashi: The blessing is that your days will be great, like this distance.