The OGC file from XCsoar (or Tophat or IGCdroid or SoftRF...) is "locked" in the sense that it includes G-records at the end (a type of encrypted checksum of the whole file), thus if you change anything in the file it won't pass the validation test. The only reason it is not "IGC certified" is because it was generated on hardware that is not sealed, so you could in theory have fed it fake GPS data or something.
Doesn't Naviter offer you an updated version of their app, to run on your new phone? Or, you can still use the old app on the old phone if you still have the old phone, even if it does not have "service". Or, do what Ryan does, set up the task in XCsoar and have it send it to the S100 and from there to the FLARM, if you have an IGC-approved FLARM. Similarly in my glider I have Tophat (on a modified Nook) connected via serial cable to my FLARM, thus I set up the task in Tophat (easy) and it sends the declaration through that cable to the FLARM.
But I agree, if you have an Official Observer vouching as to your declaration, it shouldn't also be required to have it within the flight log, which is easy to botch. But them's the rules.