"Very many specific things about my friend, her hair, her body shape, her interests, things about her that I had always thought in length about, how suddenly she died"
Presumably as many of us do now your friend had an online presence? I can imagine much of that VERY easy to check on eg Facebook or even linked in or similar, also YOUR online discussion of her, photos of the 2 of you together, your discussing your bereavement.
"She said that there had been some chatter of a new car in my household and specifically something about a BMW... our BMW died about 2 months ago and we have been going round in circles deciding if we should get a new car to replace it..." Again very easy to check what car you drive, either online or if you still HAVE the bmw in person on day of event by observation by plants or CCTV and the age of the car etc - plus YOUR response when she started talking about it, subconsciously nodding/shaking your head when she gave info etc
"We gave nothing but our name's on the door." How far in advance did you book? Because really it takes no time at all, a few hours maybe with Internet to get a lot of info
"because I am so skeptical I was very neutral and I gave little reaction" many reactions are minimal and subconscious but innate. "Lie to me" looks at microexpressions which can last fractions of a second yet people skilled in facial reading can pick up on them.
"I've a significant family date in every month for example!" Exactly! I'm the same! But in addition we have in the family 2 "bad" months when we dread unexpected calls as they're when we've lost people - but wouldn't be hard for someone to gain this info from our social media posts! Comments like "well it's bloody April (not a real bad month) isn't it! Always jump at calls then"
"but my profile is (should be) private." Privacy is an illusion these days. My mum won't "do" online banking as she "doesn't want some hacker stealing all my money" she won't have it that actually the bank will ALREADY have put all her info online she just can't access it herself! That banks have among the best digital security, but nothing's perfect and that actually if she did bank online rather than what she does - which is read her paper statement once a month! - she would know far sooner IF her account was compromised (or indeed if there's a mistake - which has happened to her a few times with certain bills ending up unpaid and she's ended up with charges etc)
It's scarily easy to access the most "confidential" information electronically. My friend's husband this is his job (digital security for a bank) AGES ago even before social media he scared the shit out of my previously naive self by being able to - with ONLY my name and dob - accessing all kinds of info about me using a normal PC and dial up Internet! This included medical data including my last prescription and when I'd collected it! That was a wake up call!!
I have privacy settings as high as possible BUT I also know they aren't completely secure and so am careful about what I post.
"She would have to have the most immense memory to remember the faces of the 6/7 people in the audience and information specific to them..." She could have an amazing memory some do and some develop their memory skills BUT she could also have an earpiece with an assistant feeding her the info.
"now she wouldn't have had time to do any research before this session and my friend said she was eerily accurate also" again - how far in advance did attendants book?
Entirely possible a "file" was created ahead of time for each attendant with not that much time required. You have no idea how many people she employs to assist her.
"she didn't have any time for the final chap!" Who YOU didn't know? He could be a plant