So my question is, if we assume that these apparitions did appear, according to buddhist thought, do we have to assume that the individuals in question have taken birth in the lower realms as preta etc?
Strictly speaking, no. Rebirth types are infinite although this diversity manifests directly in different temporary hells that people hear about that can be individual, in effect tailored by individual karma.
Ghosts as thought of in the West could be imagination, beings in hell realms, or individual hell realms, or hungry ghost realms or deva (lower god) realms manifesting through a form of clairvoyance (and there are other possibilities as well). When we see this phenomena, our particular mentality interprets what we see and we see that rather than the actual phenomena. We see what we are conceptually primed to see.
But strictly speaking these manifestations are not necessarily evidence of lower rebirth themselves.
Of course this is my opinion and you should ask a teacher this question.
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche