I really really like the first article.
And parts of the second - but I do wonder about the notion that things like children playing with toys on their own doesn't deal with jealousy?
I'd kind of say - yes and no. Depends on how them playing together is dealt with. My sis has 3 kids and sometimes outbursts of jealousy over books or toys are dealt with by the thing being taken from the jealous kid. So far as I can see, this doesn't help with the feeling of jealousy.
The kid is still upset that their thing has been taken from them and given to one of the others but they can't do anything about it. Next time, they are careful to keep away from the others with their thing or to physically fight them off if the adults aren't watching. Probably when they grow up, there will be a similar attitude toward partners!
If it was me, I'd make sure I had more versions of toys and books than there are kids if I wasn't actively supervising. And when I was, I'd make sure that the sharers were rewarded with something good for letting their siblings have stuff. I reckon that eliminating jealous is probably going to be more effective if sharing is a rewarding experience rather than if possession is a horrible one. It's kind of what I do with the dogs!
Other friends of mine have a single child. She mostly plays on her own and has full access to all of her toys at home. But when her friends come round to play, she shares quite happily. She doesn't repeatedly go through the pain of having valued possessions taken from her and given to others.
Of course, things are easier for her parents because she is on her own. They don't have a constant refereeing job to do. And when other kids are round, they are able to watch and give their child lots of praise and attention (which she loves) when she does share nicely.
Interestingly, her parents have had poly relationships in the past although they are mono now. My sis is totally a serial monogamist - she doesn't share!
It'll be interesting to see how the kids all are when they grow up.