Worfje

You can check if you have oxygen sensors on your bike when looking at the exhaust manifold (see below image).

Agreed, so it might be that the ECU you are using isn't the original one (but one from later years that expects oxygen sensors)... or oxygen sensor were applied in the police version 😉

Faulty oxygen sensor heating element, according to Haynes service manual.

The schematic gives the impression is 'merges' and 'splits', but I assume this is a (misleading) simplification in the drawing. It just wouldn't make sense to merge and split and is also connector-wise / wire-wise unpractical.

I suspect the reason for the '4 DC outputs' are due to the fact that one pair (plus and ground) is used for charging the battery and the other pair is used for powering the bikes power consumers. If there was only on pair (thus '2 DC outputs') from the regulator/rectifyer, it would be less straightforward to result in 2 pairs (which both have their own fuses and fuse values).