1) It is never an issue for Erika; in her route Kouhei gets transformed in to a "Ghoul" (Well, that probably gives a wrong impression
- a being just like Kiriha, who are basically vampire-like servants to their Vampire masters in FA, they share all the vampire characteristics, minus the bloodlust and have the random "shutdown periods" when their bodies enter brief periods of sleep) so the two essentially stay together for all eternity, and in the True End Erika gets transformed back in to human by having her 'vampiric stone' removed from within her, so that once again becomes a non-issue.
Kiriha ... well, her "happily ever after" is bound to end one day though, and is something they will have to deal with.
2) Kiriha is ancient
i have forgotten the actual age, but unless my memory is playing tricks on me, she should be just as old as Kaya (Erika's mother) whom she used to be friends with during childhood, which was about 400 years ago ... might be a bit wrong on the actual number as my memory is hazy here, but several centuries old at any rate.
3) She was turned in to one by Kaya to remain as her "friend" forever.
4) They don't really need to migrate, because they normally live hidden so long as they stay there - the whole School is actually established by Kaya over the centuries, in order to provide safe (located on island, "enclosed location", rumors are unlikely to be taken as anything serious, etc) "hunting grounds" for her "children" until they are ready to live in the human world on their own. (Her mansion is hidden somewhere in the mountains near the school). I suppose there would be the need to do so once they leave the island (And the mansion they can use as eternal hideout), but within the premises of the story there is no need to.
5) It is the grave of her parents.