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1) The OS:Generator:Photovoltaic object in your osm looks like this:

OS:Generator:Photovoltaic,
{94b0d33a-5d67-43d5-81f1-d43d8c01601e}, !- Handle
Generator Photovoltaic 2,               !- Name
{025d69f9-56d2-4b82-bdc2-f5e3cc2cded4}, !- Surface Name
{992a551d-7ee1-4dcf-98ef-a349d6248401}; !- Module Performance Name

which compared to the documentation for that equivalent EnergyPlus object is missing the last three fields, including Heat Transfer Integration Mode, which (I think) effects how the PV system interacts with the building surface heat transfer. If you look at the translated idf (out.idf) you'll see that the object has been translated as:

Generator:Photovoltaic,
Generator Photovoltaic 1,               !- Name
Surface 24,                             !- Surface Name
PhotovoltaicPerformance:Simple,         !- Photovoltaic Performance Object Type
Photovoltaic Performance Simple 1,      !- Module Performance Name
Decoupled,                              !- Heat Transfer Integration Mode
1,                                      !- Number of Modules in Parallel {dimensionless}
1;                                      !- Number of Modules in Series {dimensionless}

So the temperature calculation of the PV array is decoupled from the heat transfer surfaces, which probably explains why you aren't seeing a difference.

I'm not sure why OpenStudio is creating an incomplete object, perhaps @macumber or @Julien Marrec can provide input. The methods to assign those fields work, so until the problem is resolved within OpenStudio you could edit the plugin script used to create the PV system. For example, inserting

panel.setHeatTransferIntegrationMode("IntegratedSurfaceOutsideFace")

at line 120 of C:\Program Files\OpenStudio 1.13.0\Ruby\openstudio\sketchup_plugin\user_scripts\Alter or Add Model Elements\Add_Photovoltaics.rb and using the edited script to re-generate the PV system, the relevant field should be created and set in the object.

2) Not sure how you modeled a 'pv layer unattached to the roof', but I suspect you may have added a second roof heat transfer surface, instead of creating a shading surface and assigning the PV system to that. When I add a shading surface above the roof of your model the peak cooling load is reduced.