It's not sweet, per se. It just increases thickness/viscosity to make up for what you hoped for in OG. I remember DFH IBA being pretty bitter, in a brown ale sort of way, so I think you're safe. I'd add either .5# or 1# of Malto - I think you could boil it in about 1 Quart of water, I would guess? - do the 1/2 lb if you want to just thicken it a little, do the full pound if you REALLY want this beer to be chewy and rich.

You could always add a small quantity of molasses when you add the malto - Molasses will ferment out a little, yes, but it will leave a few residual sugars that help to create that great smoky-sugary dark taste that DFH IBA has.
I think Splenda might be too cloyingly sweet for what you're looking for, unless you only use a little.
I bet 3-4 months would be exactly the trick to take the edge off the bitterness and make this a really great beer.
