Neither 12 or 134 are considered flammables. I don't know where that could have come from. You must not be in the US, because to my knowledge Propane is NOT used as an OEM refrigerant in any US systems. I know some farmers that use it in their tractor a/c systems, but they didn't come from John Deere that way.

R/D replacement is part of a proper reverse conversion, or any conversion for that matter. The complete flushing of ALL oil from ALL components so that fresh R12 compatible oil is part of a proper reverse conversion as well.

I have NEVER had trouble reusing sensors that have been "open to the air." You have to transfer them quickly and not leave them around on the bench overnight, but there's no problem reusing them as long as they're in good shape. When I do considerable system work, I pull the old R/D with sensors attached and seal it. Then after everything else is in place I put in the R/D last and not until I'm ready to start the evacuation very soon after it's installation.