Harden never played with Simmons. They were traded for each other in '22.
From my perspective, Kyrie blew up the Nets. KD already had 1 foot out the door in July of '22 (before ever playing with Ben). He came back to give it a try. With the issues between Kyrie and Nets, blew up the team & then with the Suns ownership changing - there was an opportunity for a windfall return for the Nets.
The Embiid/Simmons issues are of legit concern. I personally see this more of two guys jostling for leadership of the team (much like the 'issues' LMA had with Dame). The other issue they had was their games didn't exactly complement each other. Both prefer to operate around the basket - so Embiid was frustrated that he was pushed further out to accommodate Ben.
On Portland's roster, I think Simmons could be a nice fit. Grant never gets close to the paint. I think Nurk operates best from the high post/elbow area, but he can also float out to the 3pnt line.