Thanks!

Many thanks for the ideas, which have got me thinking again.

Yes, Death's-head is lighter and more agile than a fully armored warhorse (I'm assuming it would also take a much larger mount to carry all of that weight).  He also has a scent and a scream that will spook if not panic any horse not accustomed to it.  Can he blood-bind this particular horse?  I don't think so, if only because it would make things more complicated, and at this point I'm trying to simplify.  He can, however, literally run rings around his opponent.   I see him as coming up behind and beside.  Someone suggested that he could go under the armor's skirts after the warhorse's 'nads, but I don't want him to throw Jame.  Rather, I see him jabbing the horse in the butt, making him squeal and bolt (illus of this horse armor shows the tail flying free; it's also suggested that the scale armor is tied on in segments).  If he does find himself head's-on to the other's lance, I can see him catching and breaking it with a twist between his horns, maybe even fighting his way down the shaft toward the rider over the course of the contest.

Jame should be his partner throughout.  Yes, she sees him as her weapon, but she can think and deduce too, even if at first DH is head-strong about direction.   As several people pointed out, she has imagination and abilities of her own.  Can she use her Shanir nature besides the blood-link to DH?  Don't know about that.  Yes, Brenwyr can rot things with a curse, but Jame has never done that that I can recall.  Maybe someday as she matures.  My sense is that mostly she sets things in motion without realizing it that lead to catastrophe on the general principle that  "Some things need to be broken."

Another piece of the equation:  There's also the warrior on the war-horse, who is not only riding as heavy cavalry against a lighter opponent but who also is festooned with weapons.   I've just about decided who he is.