We can not say for sure that Krishna 'used' Ghatotkacha as a sacrifice in the war. All the warriors volunteered themselves.
Yes Krishna rejoiced when Ghatotkacha was slained. That event lowered the invincibility of Karna by great extent & now he could be slained.
Sanjaya said, 'Beholding Hidimva's son slain and lying like a riven mountain, all the Pandavas became filled with grief and began to shed copious tears. Only Vasudeva filled with transports of delight, began to utter leonine shouts, grieving the Pandavas. Indeed, uttering loud shouts he embraced Arjuna. ... Vasudeva said, Great is the joy I feel. Listen to me, Dhananjaya! ... The man does not exist in this world that could not stay before Karna armed with that dart and looking like Kartikeya in battle. By good luck, his (natural) armour had been taken away. By good luck, his earrings also had been taken away. By good luck, his infallible dart also is now baffled, through Ghatotkacha. [source]
The heir of the throne was always Yudhisthira or subsequent PAndava, because it was their kingdom which was snatched. And it was never Ghatotkacha, as he was not considered Kshatriya due to his maternal blood line & upbringing. Due to his inherent evilness from "RAkshasa" qualities, Krishna was willing to kill him, had he survived the great war.
Hidimva and Vaka and Kirmira have all been slain by Bhimasena. All those Rakshasas were endued with might equal to that Ravana and all of them were destroyers of Brahmanas and sacrifices. Similarly, Alayudha, possessed of large powers of illusion, had been slain by Hidimva's son. Hidimva's son also, I have slain by the employment of means, viz., through Karna with his dart.If Karna had not slain him with his dart in great battle, I myself would have had to slay Bhima's son Ghatotkacha. From desire of benefiting you, I did not slay him before. That Rakshasa was inimical to Brahmanas and sacrifices. Because he was a destroyer of sacrifices and of a sinful soul, therefore hath he been thus slain. [source]