Multiple Power Supplies Powering Long LED Strip - Proper Way to Ground
There is no need to tie ground as per the orange line, in fact it can be detrimental as you are effectively creating a long ground loop.

To understand exactly what is going on you should design and simulate your circuit with small resistors on both ground and power between each LED, like others have suggested.

Edit after changes to question 22/7:

I would recommend you connect the system as follows: