That "increasingly large area" would fall entirely inside the fireball (and likely within a few-meter radius), and the detonation would basically indistinguishable from it all going off in one go.

Disintegrating the device is a serious problem for nuclear bombs, because when they break themselves up, they leave otherwise good material unused. For an antimatter bomb, splashing the AM uncontrollably would, if anything, speed up the initiation. (After the AM and the normal matter around it is ionized, the particles home for the closest thing to annihilate with. AM basically does all the work of the weapons designer for him.)

Hmm, how about using two nukes to create light pressure from the two ends of initially split charge of hydrogen and anti-matter hydrogen? Or, however challenging by itself - creating spherical cavity in the hydrogen bomb to push normal matter to antimatter in the cavity for awhile.