Ok. Can I just clarify that personally, I am not talking about UP, I am talking about the philosophy of consensual living as applied to parenting. It is different, but UP is what led me to it.
I would stop dangerous behaviour by whatever means necessary. I will hold their hands to stop them hitting, I will put them in another room if necessary, but I make it as clear as possible that that is to protect them/the victim until they've calmed down enough to actually talk about it, not as punishment. All of mine have been through throwing/hitting/pinching phases as toddlers, and it's been very unpleasant and confusing to deal with, but they've all grown out of it so now, with DD3 and DD4, I just accept they'll grow out of it, keep telling them it's not ok, keep suggesting other ways of dealing with anger, and keep protecting their victims.
Your examples:
--coerced my 14 month old in order to change his nappy. Me & his dad had to physically restrain him in order to change it as nothing else worked
Yes, I struggle with this one a lot, particularly if it's dirty. Buy different nappies that are easier to change? Whip it off when they're standing up and only lie them down to put the new one on? Put a tv programme on for them to watch while you do it? Give them someting nice to eat while you're doing it? Take it off to put him in the bath (or do something else he loves) and put the new one on in a different situation ISYWIM? I don't think toddlers do have to be coerced for nappy changes, but I do agree that, for me, it's one of my biggest challenges trying to find a way of doing it that doesn't coerce them and I frequently give up and just do it as quickly as possible .
--I stopped him in Starbucks from playing with a paper napkin because he was tearing it in tiny pieces & throwing it on the floor. He didn't like that I took it away from him.
Why did you take it away from him? Was it really a problem? Could you just have cleared it up afterwards? Given him something else to do before taking it away? Was he bored? Was it time to drink up your adult drinks and leave?
--I coerced him in order to get him dressed (to go to Starbucks). He didn't like that either although he did cooperate finally, but I wanted to do it relatively quickly so a bit of coercion was involved.
Could you have taken clothes to put him in when you got there? Let him go in his pyjamas? You're lucky with him being so young - you can get away with pyjamas when you're out a bit easier . But in reality, there's no problem with wearing pyjamas out except getting cold. Bring a jumper and warm socks?
--He was playing with the kettle, managed to remove it from the shelf & was dragging it with him. I took it away & said it's not something he can play with. He didn't like that either.
Give him something else interesting to play with? What was dangerous about the kettle? Could you have made it safe for him? Then ensure it doesn't happen in future by keeping it far back on the shelf?
I don't know what I think about your DP's situation. Were there two parents? Couldn't one have gone to find the others and said that the DD and other adult wouldn't be joining them or would catch them up? I agree it's very rude and inconsiderate and not teaching their DD anything about co-operative living, just about selfish living. Fine if she prefers to write and parents are happy to sit with her while she does, but that's not a solution that suits everyone because other people involved in the situation are unhappy.