I don't know the details of Henry IV, so I can't speak to why he was moved to consecrated ground.  But excommunication doesn't necessarily entail automatic damnation.  It severs a man from membership in the Church (in its most severe forms at least, although not all excommunications entail loss of membership-- a toleratus is deprived of the right to sacraments but remains a member), but just as a catechumen may be saved via perfect charity despite lacking membership, so too might an excommunicate-- either owing to his excommunication being unjust in the first place (e.g. Joan of Arc) or him making a perfect act of contrition yet dying before being able to be received back into Church membership-- avail unto salvation.  This is described with various expressions by theologians, although member in voto seems to be the preferred expression given its use by the Holy Office and its promotion as a term by Mgr. Fenton.

I see, thank for your answer. As with everything in the Church, nothing is quite so simple as it seems. There's always deeper nuances to every issue.

Interestingly, Dante (who was a theologian whose theology was eclipsed by his singular poetry) placed ALL the excommunicates in purgatory.

For him, all the excommunicates were -so far as the excommunication only is concerned- saved.

Huh, I never knew. I always assumed excommunication placed you out of the Church entirely, in a way that EENS would damn you. I guess you learn something new every day.

.
Pope Pius taught excommunication severs one from external membership.  But MCC also acknowledges the possibility of another type of union, later in the docuмent.  I would take this to be membership in voto, given later developments; membership in voto is how we would describe the saved catechumen, and I think this applies to the excommunicant who is in the state of grace (Bellarmine argues exactly that, if I recall correctly, though with different terminology).  Someone who enjoys a special and real internal union with Christ and the Church owing to their having been justified by grace and the requisite supernatural virtues, but who nevertheless does not enjoy the bond of external union.
.
Dante was described by Benedict XV as a student of St. Thomas's and he described Dante's work as thoroughly informed by Catholic principles and teaching.  Benedict's encyclical on Dante was pre-MCC, true, but if there was any serious error in Dante's work (i.e., an actual contradiction with Catholic ecclesiology or soteriology) I doubt an encyclical would be able to praise him the way that Pope Benedict's did.

The Church is not infallible in meting out penalties to individuals, as such actions lack the universal sort of scope upon which infallibility hinges.  The same goes for other legal proceedings which do not concern the universal Church, like declarations of marriage validity or invalidity.