1)
Who are the "Mesei Mispar" who will return?
1.
Radak: Yirmeyah, Baruch ben Neriyah and a small number of other Tzadikim among them.
2.
Malbim: They are "those who escape the sword" mentioned before this.
2)
What is the meaning of "v'Yad'u... Devar Mi Yakum Mimeni u'Mehem"?
1.
Malbim: If their word will be fulfilled against Me, or My word will be fulfilled against them. I.e. we will see who has the ability to fulfill his word - if they will fulfill their vow, or I will fulfill My decree.
