CODE P0193 all that was left?

OK that code is for the fuel rail pressure sensor. With the engine at idle toss a scan tool on it and see what the PCM is asking for in fuel pressure VS what the sensor is showing as fuel pressure. Are the values different? Yes - test the voltage at the sensor between pins 1 and 3, you want to see 4.8-5.3 volts. If the values are wrong and the voltage is OK, replace the sensor.

Unplug the sensor and test the voltage supplied at those two pins with the key on, should have bias voltage to those pins. If the voltage is good then it's likely a bad sensor or wiring to the PCM.

That is a fuel pressure sensor implausible code. IE it's reading higher or lower pressure than the PCM thinks it should. Might want to look at the connector on the sensor and see if the sockets are loose on the pins or if there is damage to the connector. Could even just be a bad sensor itself.

P0355 is a cylinder 5 coil implausibility code. It might share a circuit with the pressure regulator but the diagrams I have doesn't really show one. It does show a connection to the crank sensor and the oil temp sensor. No real way to test the pressure sensor other than replacement as MB keeps that info in the dealer network.

OK that is an odd code, Basically says that the spacing of the teeth on the crankshaft for the crank sender is wrong, or more likely there is a wiring fault to the sensor. You mention you only have 3 wires but there should be 4 and one is shared with the crank sensor.
In the earlier image, B4/15 is the oil pressure sensor, B70 is the crank sensor and B1 is the oil temp. Since you are showing a code for the pressure and the crank, I'd look at the shared parts which is pretty much the 5 volt feed and the PCM, but as your 5 volt feed was bouncy I'd check it right at the PCM pin 44 If it's steady there then I'd unplug the crank and fuel sensors and see if it still bounces with them unplugged. Could simply be a bad sensor pulling the bias voltage down.

The code you have is fuel pressure sensor implausible. The pressure it is sending is higher or lower than the pressure it is requesting it falls into this tree of codes -
The value of sensor B4/15 (Fuel pressure sensor) is implausible. (P0191)
Fuel pressure is too high. (P0191)
Fuel pressure is too low. (P0191)
Fuel pressure control : The lower limit value is reached. (P0191)
Fuel pressure control : The upper limit value is reached. (P0191)

All of them start with testing the two fuel pump control modules for faults. Then they want you to use the scan tool to select a pressure for each module in turn and use the sensor reading to see if the modules operate correctly. So basically you turn on one pump to say 50 psi, then look at the sensor, it shows 50, now select the other control module at 50 and check the sensor, it reads 25, that pump module is the issue. Probably could do that using a pressure gauge on the rail in place of the sensor, then watch the gauge as you cycle the pumps on and off. If both pumps read the same then it's likely the sensor itself.
Both pumps are in the same pump module, there is a level sending unit and strainer sock on one side and the pumps are both on the same side with one pump connected to the other side with a hose in the tank.

You should be able to power them individually using a scan tool as they can be controlled individually. So if you turn on pump 1 and get a correct reading, then pump 2 and get the same reading the pumps and control modules should be OK. The issue so far Is that nothing I've seen shows the actual fuel pressure, just the range the sensor reads and tells that you compare the requested pressure to the sensor read pressure.

The fuel pressure sensor is located at the front on the left side of the fuel manifold.

The fuel pressure sensor detects the fuel pressure in the fuel manifold.
The fuel pressure is evaluated by the ME-SFI [ME] control unit (N3/10) for actuation of the left and right fuel pump control units (N118/3 and N118/4) (except model 216, 221) according to requirements or the left fuel pump control unit (N118/3) (for model 216, 221).

Function
The fuel pressure deforms a membrane, which affects a potentiometer. This alters the resistance value of the potentiometer and influences the signal voltage (about 0.3 to 4.8 V) in such a way that it serves the ME-SFI [ME] control unit as information about the fuel pressure.

The pressure sensor measuring range lies between 0.5 and 10 bar.

The fuel temperature is also detected by the fuel pressure sensor but not evaluated by the ME-SFI [ME] control unit.

Yep, compare the reading from each pump to the requested pressure in live data. So say it shows the left pump at 5 bar requested but the live reading is 2 bar. Then you check the right pump and it requests 5 bar and you see 5 bar, there is an issue with the left pump. Also compare the live data reading from the pressure sensor to an actual pressure gauge. If the mechanical gauge reads 5 bar but the live data reads 3 well you know the sensor is an issue.