OK, then try this.
I understand your 920 is about to be sold or given, so use it a last time like this :
a) Flash it under WP8.1 (WDRT)
b) You don't need to restore any backup. SMS restoration is independent. Just log into your MS account, say you start from scratch
c) Go to messages, settings, and enable "restore from anytime".
d) Go to settings, emails+accounts, and sync your main account as long as it says "not synced". This will force the sms to be restored
e) When you are sure that all your SMS are back, install windows insider app, and the latest build
f) once migrated, again, don't install or restore a backup, it's not needed. Go to the store, update all apps including messaging
g) in Messages, settings, disable/re enable sync across devices, anytime.
h) wait for a while
i) you need to let the app (messages) open, as it seems to sync when you open the app.
j) you could monitor on your 950 if new messages are coming back.
The whole idea is to restore all previous messages thanks to WP8.1, migrate them to the post 10572 standard, and once there, all your W10M devices should sync correctly.
I did the same and it worked. You mention 10549, but as I said on the "sms restore" thread, the new standard of message backup has been introduced with 10572.
You may notice that you'll have duplicates or triplicates for some messages, but at least you'll have them all.
The new standard seems to sync differently : you'll recover by thread (all your messages with contact X, then contact Y, etc.)
You'll see that the new W10M restoration is a lot faster than on WP8.1, but I had to "force the migration" by following the process mentioned above.