Is a “life between lives” hypnotic regression a risk factor for allowing intrusion by dark spirits or extraterrestrial psychics to talk through the person and misdirect them and possibly lead to ongoing possession?

This is very true. It is an open-ended outreach of the broadest possible kind because people have no innate awareness of how to make such a connection with precision and precise focus, so there will be a broad outreach in all directions and this will always be noticed by the extraterrestrials who will come to see how it might be exploited and things will proceed from there. It might just be a routine idle interception and insertion of their voice into the session to plant a seed for the future, but these things are never frivolous because they have many, many quite serious projects ongoing and have no spare time for folly. It is very serious, and deadly serious, work to them.