There is nothing I can see as non compliant. For many SS materials sec IX is silent as far as essential variables go for purge.
You could lay a plate flat on a table, weld one side, flip it, backgouge it (or NOT) , weld the second side and write a WPS for 1/2" Schedule 80 open roots with or without backing and with or without backing gas.
Code compliance as far as Sec IX would be fine. What you actually put into a WPS will provide the user with the information they need to create the weld in accordance with your companies or customers requirements.
My answer to "My questions is....by substituting argon with the Solar Flux, will my WPS be Section XI compliant" is yes (Sec IX)
Currently there is no essential variable that addresses to use of flux either applied to a joint. It is addressed in filler metal Type 404.23. So you can use solar flux without concern however changing to a flux cored or flux coated rod would be different.
Individual project specifications may dictate your ability to use solar flux. I would suggest preparing 2 WPS's . One with Solar flux and One without. Be specific in the documents to how it is done. Flow rates are one thing, checking for O2, vent location, backpresssure, can all affect the appearance of completed roots.
Solar flux procedures should have instruction for how to mix, care for handling of joints after its applied, cleanup of internal surfaces if required.
This is only my opinion based on what I have read. Feel free to correct me and point me in the right direction if needed. I have searched through all of the interpretations and nothing has been submitted that I could find . That includes Sec IX and 31.3 .
Here is a solarflux article from a few years ago.
http://www.solarflux.com/Graphics/web%20graphics/WDF_SOLAReprint.pdfGerald
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