Taylor, congratulations on your new house. And I'm sorry to hear about the Italian Arum! It certainly can be a nightmare. Unfortunately, if you want to go strictly organic, you'll need to dig it out, realistically over several years. 

Here is an article from Oregon State University: Italian Arum | solvepest. The longer you let it go the worse it gets. It's really a horrible plant. 

I couldn't find much on metsulfurn-methyl but this is from the Nat'l Library of Medicine: "Metsulfuron-methyl is a residual sulfonylurea herbicide that kills broadleaf weeds and some annual grasses. It is a systemic compound with foliar and soil activity, that inhibits cell division in shoots and roots. It has residual activity in soils, allowing it to be used infrequently but requiring up to 22 months before planting certain crops (sunflowers, flax, corn, or safflower). It has very low toxicity to mammals, birds, fish, and insects, but is a moderate eye irritant."

There is a concern about applying it near the roots of your cedar. Here is something from University of Florida: FOR332/FR400: Metsulfuron-Methyl-Containing Herbicides Potentially Damaging Ornamentals when Applied to Turfgrass. 

Another from the University of Florida: Some Herbicides Are Killing Trees - UF/IFAS Extension Broward County.

This is from Ask a Master Gardener of Washington State: , “There are no effective ways to control the plant, and it is unresponsive to herbicides. Manual removal is difficult. If removed, all parts of the plant should be placed in the trash and not your compost bin.”

I know none of this is good news. It is a very difficult plant!

I wish you the best. An enjoy your new home!

Susan, Yamhill County Master Gardener