I asked around on the Game of Thrones Embroidery group on Facebook as to what other people used to glue their embroideries and the universal answer was watered down fabric glue.
So I used about 1/3 water to 2/3 Sobo fabric glue, mixed it together, turned my embroidery over and liberally coated the entire back with it. Once it was dry the organza was VERY stiff and almost rubbery, flexible but not very fabric-like any more. I cut into it and tested it a bit and found it was very sturdy and wasn't going to shred at ALL.
So I cut very close to the embroidery, leaving on a small margin around things, making sure not to cut any threads. Be forewarned - your fabric may behave differently, do some tests!
Back (oh the horror!!!)
And close up so you can see the margins.
Then I pinned it securely on several points on the sleeve where I wanted it to go then carefully whip -stitched it with one hand inside the sleeve to keep things from puckering, using tiny stitches and black thread and occasionally going into the design. It's not tacky but is stiff enough it's a bit tricky to sew through, but it also doesn't feel like it's going to fray at ALL. Then I started adding the surrounding beads. I'm pretty happy with it.
Added bonus, I may just be able to do this same thing with the embroidered bird fabric I plan on using for Cersei's aqua bird gown. I did a test with these and they seemed to hold up well, despite being on silk taffeta and not organza. I would then applique them and add more embroidery around the edges to cover the yellow bits.