Ok - I think the light has dawned! The SDK you should use with FSX SE is
Prepar3D v1 SDK 1.4.4747.0
see the link
https://www.fsdeveloper.com/forum/threads/can-i-use-the-sdk-tools-addons-with-fsx-se.444148/ and where to download I have just done so and installed.
1) Install the SDK and in MCX Exporter BGLWriter settings update FSX BGLCOMP path to point to the bglcomp file in the SDK
and in MODELWriter settings update FSX xtomdl path to point to xtomdl file in the SDK
2) In the MCX FS related settings keep the modeldefpath pointing to the P3D v5.3 SDK
With this setup you are using the P3DV5 SDK to interface with the Blender P3D/FDSX toolset to produce .x and .mdl files. (You will use .mdl file output if you have animations - when we get to SODE controlled animations - does FSX SE support SODE?? You can check this when you have the time).
As you rightly said, FSX SE does not support PBR textures, so we have to indeed use SPECULAR.
In the RAR file is a specular version of the LightPathway Blender file. There is also the resulting .x file the toolset produced. I inputted this .x file to MCX and saw the transparent shade.
I then successfully exported the model from MCX as an FSX .mdl file (buy having MCX correctly pointing to the P3DV1 SDK). I then imported the FSX.mdl file to MCX and again saw the transparent shade. SUCCESS - I hope. You need to see if you can successfully convert/place the model into FSX SE.
Observations: In Blender using specular textures, the transparent shade has to have Blend = Alpha Blend and further down (past the texture definitions) under Framebuffer Blend, SET Transparency button must be pushed - the opaque shade turns to a rather cloudy transparency. If you were to work on the filament or Empties you would wish to hide the shade first. By the way, to get the transparent texture for the shade it is simply a Photoshop light blue on a transparent canvas with the transparency of the blue layer set to 10%.
I really hope the above does the trick. Have a good inspection of the Blender file to understand how it is constructed and let me know how you get on.