DALI Signal Voltage Range and Power Supply connection confusion with WAGO
First you should check that your scope settings match your probes regarding probe attenuation eg 1x, 10x. Second you should detach any other loads from the PSU to see if voltage goes back up when unloaded.

Q1. Maybe the PSU module 787-1007 is damaged due to it being used as a DALI bus power supply when it is not such a thing - it is a power supply for the 753-647 DALI multimaster module. See Page 14, section 3.1 of the 753-647 datasheet  - "These power supplies do not provide direct power to the DALI bus, but only indirect power supply via the DALI Multi-master Module". This means you connect the DC output of the 787-1007 to the U pins on the 753-647, and take the DALI signal from that module. See section 6.3.2.2 for wiring diagram.

Q2: The current limiter isn't shown in that diagram but is in the 753-647 module.