What is the reason for additionally having the LEDs connected with the ground pins on the pi board and (b) what is the reason to have the 18A PSU 5V+ connected to the 3.3V of the Pi board (i thought the Pi is now being powered by its own charger)?
the reason is that you never can have to much grounds, the more the better.
The PI is not being powered on PIN1 but it powers up the logic level shifter on LV input.
the converter then "knows" what it has to do with the channels you can say. Between LV and HV the converter sets his output on the channel.
A level converter you can also use to increase instead of decrease data lines, just connect the device at the other end. Its called bi-directional.
What is the reason people here do not recommend to have the Pi powered by the same PSU as the LED's. Ideally I would still like to work with just the one PSU. What Pins on the Pi would i need to hook up with a positive and negative wire from the PSU to power the Pi without the USBC charger (just for my own understanding)?
I doesn't really matter that much and its more a preference to power individually or everything together.
Most important thing is this; especially when used more then one PSU then grounding becomes life important between the devices and the PSU's.
ledstrips are really sensitive for bad grounding or bad PSU's and connections
your last question is a good one, yes you can power the PI directly on the powerPIN 5volt and GND on the PI, u can use PIN 2/4 for 5volts and 6/9/20/25 and so on for GND. Should u power the PI like this? thats a good question because in this way you evade the electronical circuit that protects the PI for shortage/mallfunctioning and and such. Do i think this is a good way to go? yes and no, first i would try with original USB power port of the PI>> if that would all fail then yes then i would use directly the power PIN.
I have it connected with only one really powerfull PSU of 35 Amps on 5volts, using the original USB port of the PI3b on that same PSU.
the only problem is now that with the PI4 the USB port expects' a high current it can draw, so when thats not possible because of using bad PSU or whatever then you get undervoltage warnings and a PI4 that wouldn't run good.
For this i make an exception and say then yes, power the PI4 on USB-C with a SEPARATE PSU