I know one of them took me ages to get in the Playrooms

It was suspended in a cage on the other side of the room, but it was not a straight jump back. You had to get him into jumping position where his feet are up against a wall type thing, the you have to push left or right at an angle and then jump, This makes you jump backwards to either the left or the right.

The standing True Form I can vaguely remember, I haven't got that far yet though, just left the Cathedral of Pain and done my first trip to liveside and seen one of the serial killers and gone back to Nettie. Collected the first piece of L'eclipser so still a fair way to go. If you are using the guide read a bit further on than where you currently are, sometimes it goes back to a bit you have passed already. I do believe if my mind serves me right that some of the True Forms in cages actually stay there as they don't have a Dark Soul imbedded in them to give them life.

Have you collected the Dark Soul at the top of the Bloodfall at the very start of Deadside Marrow Gates section, easy one to miss.

Have a look at the Clean Up sections in the guide, there are 3 of them and usually guide you to things you may have missed with instruction on how to get stuff, I also believe if I remember right that you need to have the Calabash Voodoo Artifact to get a few of the last remaining souls, this is the last item you actually collect.