So I was at a loss on what to do. At first I figured, I'd transplant it to some ant-less soil, but then I thought of trying something else. It was 4 PM and the sun was pretty strong. I decided to drown all the ants!! Die ants! Die!! How dare they attack my 3 for P100 plant! So for about half an hour, I submerged the plant in water. It's a plant. It can't possibly die with too much water for just half an hour. And the drainage was good too. After the deed was done, I moved it to another location (hopefully with less ants).
Today, it's looking good again. Of course I pruned off about half the leaves but I see signs of recovery. More importantly, I don't see any ants in the soil. Meanwhile, after deadheading the purple petunia, it also looks like its about to blossom again.
My mums have been quiet for a while. After struggling for a couple of weeks, it looks alive, but still pretty bare. I've applied some fertilizer so it would bloom again. For a P30 plant, mums are high maintenance!