For starters PLEASE don't bandy about "ocd" in the way you have - it's clear you don't understand the condition and it's offensive and insulting. I have dx ocd, it's currently got me housebound, 4th time that's happened, 18 months so far.
Rigid black and white attitudes to certain things is normal especially at this age.
You seem somewhat obsessed yourself with trying to find a medical label for normal albeit spoilt behaviour of a 2 year old. Are you currently training as an hcp? Or in some kind of psychology course?
Quite honestly as not only a parent, an aunt and someone who's minded other people's kids for over 30 years in various capacities I think you pandered and you need to stop doing that. Yea you pick your battles but this is one that needed picking, there was a seat available next to you on the other side, she'd left the seat for long enough for it to be considered available and really it's no harm to her to have sat in the other seat. Plus her speaking to people rudely REALLY needs nipped in the bud that's completely unacceptable.
Db may currently be too extreme on the disciplinary side of things but you seem to have veered too far the other way - which is just as damaging to be honest.
Children need boundaries and to know you're in charge. It makes them feel safe and allows them to develop into secure happy people.
"But I'm not like that at home only at special public occasions" and no doubt dd has sussed that and is using it to her advantage to a degree.
Consistency is key. You'll only confuse matters if the rules at home are different to the rules elsewhere.
What you're actually avoiding is embarrassment - well your responsibility as a parent trumps any embarrassment you may feel, plus other adults especially parents will think more of you for disciplining her than they will of you if you're pandering. Your db has had the guts to say it, I doubt he's the only one thinking it.
"but keep the peace at the event. Is that pandering then?" Yes! How will she learn to behave well at such occasions if she knows you'll let her away with behaviour then that you wouldn't otherwise?
"She said to DB "that's not your chair, that's my chair- sit over there". Isn't that just how 2 year olds talk?" Wow! No! It's really not!
For that alone I'd have taken my dd (or indeed any other child I was responsible for) at that age aside and firmly told her it wasn't good or kind to talk to ANYONE like that!
Her "having a point" does NOT negate good manners!
I really do think you've gone too far to the soft side of disciplining her and need to adjust.
If anything it's more important to behave with good manners and no tantrums in company than it is in the privacy of your own home.
You need to get a handle on this ASAP or you will likely have real problems with her behaviour at such events in future.