I don't think you would need to worry about "fumigating" the soil for something so un-established (but am not an expert on this)......and I definitely don't think you would need to worry about even replanting.....I agree with Hman's perception and would be willing to bet that this tree will re-sprout. Am continually amazed at how sturdy trees (and fruit trees) are.....I don't believe I have ever killed one by pruning.....am also amazed at how much people are worried about killing them by pruning too much or at the wrong time, etc, etc. Trees need care, yes, but they are also tough.....often tougher than we think. The story of one of our oldest pear trees, the Endicott Pear, is almost a comi-tragedy at just how many times this pear tree was destroyed......only to come back to life again.....year after year....for literally hundreds of years. Try getting a link here. http://en.wikipedia.org/wiki/Endicott_Pear_Tree

I don't believe Voles care what the variety of plant roots they eat. I have had every type of tree as well as herbaceous plants killed by Voles. I'll go out and find a small tree leaning over or a plant pulled down into its hole and know exactly what is going on. I have planted Virginia Bluebells three times, only to have Voles destroy the entire bed. The last time I planted using permatil in the planting hole, but the little bas... still got them. The sharp edges of the stone did not deter them. Some people plant every thing in pots or in wire mess, but I have little time and too much other things to take care of.

With that red-twig dogwood in the planter I'm thinking maybe you like red twigs, making a coral bark maple seem like a candidate. 'Beni kawa' is more deeply colored than 'Sango kaku', might look OK there.

Maybe the fig roots are doing you a favor by sucking up the water from the leak, if there really is one. My neighbor had a drain pipe from his shower that unbeknownst to him became detached from the shower pan, so all the water from every time he took a shower was draining under the house into the crawlspace. It caused the soil to get so soggy that one end of his house sunk three inches, making his house unlevel, which cost him big bucks to fix. BTW, I have a Chinese Elm about 20 feet away from a rental house in the backyard that's roots grew under the house about 30 feet wide and came out another 15 feet in the front yard, for a total length of about 65 feet. My neighbor had a Eucalyptus that had roots 50 feet long that grew under the fence and were sucking water out of my garden.