Will this work (optocoupler over shunt)?

[spec]

Your long reply above indicates that you haven't taken in a word that I have said.

Please keep out of my hair and I will do the same to you.

[Simon]

spec - "Attached below is the schematic for a bulb current monitor with a voltage overhead of 50mV."

Even less voltage drop than the previous circuits but is the optocoupler and the second 12VDC supply necessary? Seems like the two halves could be combined with the comparator driving Q4

Of course, but that is not what the OP has asked for. We can only assume that the OP has a reason for having two isolated supplies. The resolution would be to ask him.

Another aspect of simplifying the design is to ask what the relay is for. Maybe that could be eliminated too.

The OP has used a simulation program te try a concept out and has likely been a bit "lazy" with circuit diagram conventions. They should perhaps be specific as to whether or not an isolated supply is required as if this is for use an a car then it will be several quid for the likely unneeded DC/DC converter.

[Zero999]

spec - "Attached below is the schematic for a bulb current monitor with a voltage overhead of 50mV."

Even less voltage drop than the previous circuits but is the optocoupler and the second 12VDC supply necessary? Seems like the two halves could be combined with the comparator driving Q4

Of course, but that is not what the OP has asked for. We can only assume that the OP has a reason for having two isolated supplies. The resolution would be to ask him.

Another aspect of simplifying the design is to ask what the relay is for. Maybe that could be eliminated too.

The OP has used a simulation program te try a concept out and has likely been a bit "lazy" with circuit diagram conventions. They should perhaps be specific as to whether or not an isolated supply is required as if this is for use an a car then it will be several quid for the likely unneeded DC/DC converter.

Well they've used a relay, as well as an opto-coupler but only one is necessary for isolation. Both a relay and opto-coupler could be used together to provide the sum of withstand voltages of both parts, but this is only 12V and it's generally cheaper to use only one isolation component, with the appropriately rated isolation voltage.

[MK14]

The source of the original circuit design, seems to come from here (speculation, I could be wrong, but it seems amazingly similar):

https://www.electroschematics.com/11918/auto-12v-bulb-failure-warning/

The Opto-isolator, seems to have been put in (as regards the link, above), for future options, rather than because it is needed now.

I presume, the OP, based their circuit on the above linked circuit, and have added a relay and changed it, for reasons unknown. I can only guess at what they are trying to do.

For anyone who is curious, schematic copied here (from link, above):
Then below it, is what the final unit would look like: