Two separate problems.
One is power and brakes. This is simple and U-Haul
can do this.
Two is the euro trailer wiring and the 7 wire to four conversion.
European cars and trailers have bulb out warning and have separate circuits to the right and left sides for running lights and turn signals etc, They also have separate circuits for the brake and turn signals.
US trailers have combined turn and brake signals where they are not. Three wire to two converters are required and you can't just tie the wires together you need a converter to keep from screwing up the car's wiring and the brake and turn in the trailer working properly.
Most require a powered converter since the car is lookig for the proper loadsfor it's light
bulb burn out features to work.
If your Volvo guy does not know how to do this without rewiring the trailer you need a different guy for the trailer wiring.
The Volvo may have trailer stabilization that would function if the proper Volvo module is installed like the VW. This module would still need a three wire to two wire converter to function properly.
Note that the left and right tail lights
etc are on the list along with the separate brake and turn lights.
The powered converter uses the brake signal and the turn signals to create the combined brake/turn signal so that you can have brake and turn signals at the same time. That powered converter keeps from loading the light circuits so that that pesky bulb out detector will work.
Here is a pictoral of the hookup for the Euro system.
If you convert your trailer it will be incompatible with US TV setups.
Convert with the three to two gizmo and then add in the power and brake systems.
Here is a link to the Erich Jaeger kit to install the module required for post 2007 Volvos so that the system will work with the ESP system
If this module is installed then the car's stabilization system will detect and function with the trailer as required by law in Europe since 2007.
After the kit is installed then you can use the unpowered three to two wire conversion to drive the trailer.
Here are the instructions for a converter like I used with the VW:
Trailer Hitch Instructions For DrawTite, Reese, Hidden Hitch & More - etrailer.com
If you do the above then the ESP will function for the trailer as required if you were in that crazy Euro Zone and the trailer lights will work with US trailer wiring.