1)
What is "Ma'aleh Vamah"?
1.
Radak: It is one who offers an animal on a private altar.
2)
Why does it mention cutting off those who offer to its god?
1.
Malbim: In Yeshayah (16:12), it says that when Mo'av sees that it has no strength to fight the enemy, it will want to pray to its god. It will not be able to.