1)
Who is "Meri"?
1.
Radak: This is an adjective (rebellious), like "Al Tehi Meri" (2:8).
2)
Why did He say "Rav Lachem mi'Kol To'avoseichem"?
1.
Radak: All the abominations that you did, until you brought Arlei Lev and Arlei Basar to be in My Mikdash, to desecrate My house. This was said to Bnei Golah ? what they did before they were exiled, and what those who remain in the land are doing.
2.
Malbim: He rebukes them for letting Bnei ha'Nechar serve. It is already a great sin to let them enter the Mikdash, even without Avodah. How do you increase to sin via letting them serve? This is more than all your abominations!