Regarding Trident,
I don't think it was a mistake, nor do I think it is a coaster.
Whenever Frontier announced returning rides for PC2, they used the name of the ride in PC2. Like when Mind Melt (Vekoma SLC) returned in March, they didn't say Returning ride: Mind Melt, but they said Returning Ride:Vector-Inverting Suspended Coaster, which is the PC2 name for this ride. Pathos III and Scizzer still kept their names in transition to Planet Coaster 2. Which means, whatever returning flat ride is coming this month, Trident is it's new name.
Now what that flat ride is, I have no idea. I do know that the ride names change when they are de-themed ie VIctory>Upswing.
I guess we will find out soon enough.