I suppose you meant to write 'perhaps Adirath and Radha were not alive after the war'?
In the KMG text, it said that it was Bhima himself who told Hidimba that he'd stay w/ her until she had a son, and then he would leave. It's not Kunti who put that condition. But yeah, the reason for that was that a Rakshashi couldn't live w/ them in Indraprastha or Hastinapur, as it would probably frighten the citizens. But when the Pandavas were in exile, they could have lived w/ Hidimbaa, no? As it is, Ghatotkacha and his followers often came to them to help them, particularly carry Draupadi & the Pandavas around when they were tired. So they could as well have lived w/ them.
Another thing - Bhima left Hidimba once she had a son to take care of her. But once Ghatotkacha was dead, after the war, shouldn't Bhima have tried making some other security arrangements for Hidimba? Or was that up to Meghavarna? Incidentally, KMG's Mahabharata mentions only one son of Ghatotkacha - Anjanparva. This son was killed on night 14 by Ashwatthama, shortly b4 his father's own death @ the hands of Karna. If Meghavarna survived, he was his brother. As I mentioned to LJR, Barbaryk is probably fictional, for all the reasons I mentioned there, and also that none of the mainstream texts seem to mention him.
I didn't get what you meant by 'Iravana' being meant to be sacrificed. Ghatotkacha was - to spend Karna's shakti*. But Iravana was killed by another rakshasha Alamvusha, who was the brother or son of Bakasura. This rakshasha warrior created havoc in the Pandava ranks, and only Abhimanyu repelled him once, but everyone else, including Bhima, was unable to defeat him. Ghatotkacha killed him in battle on day 14, while Arjun was invading Jayadrath's kamal-vyuha.
My point above - as you mentioned, that gem could be used on more than one person, and Uloopi used it for her husband. But her son died even b4 that. Couldn't she then have gone to Kurukshetra and revived him? And while she was @ it, maybe revive some of the other Pandava fallen?
* Ashwatthama had 2 weapons as I mentioned in the last page while discussing Barbaryk. One was the Narayan-astra, which he used on the Pandava army after his father died. It was unmatched, and the only way to avoid its ill effects was to lay down all arms & resistance and surrender b4 it. Krishna pointed that out to the Pandava army, and most of them were saved. His other weapon was the infamous Brahmashira, which he used on Uttara. As it is, Ashwatthama fought Ghatotkacha after killing Anjanparva on night 14. Given that Ghatotkacha was unconquerable, he could have used either of these weapons - maybe the latter - to kill Ghatotkacha, and it would have worked. I don't see why Karna had to be the one to spend his shakti.