Did I Screw This Up For 9th Priming?

Unfortunately, I don't think there are going to be any better scrolls than primes. I would recommend using the 15% spell trace scrolls. If you manage to get both to fail, attempt to blow up your sword by star force enhancing it beyond 12 stars. When the equip blows up from star force enhancements, the stars will get wiped. Then you can clean slate the sword and attempt to prime scroll it.
Also, don't enhance weapons before fully scrolling them, since they only gain 1 + floor (attack/50) attack per star, so your sword essentially has 7 less attack than it could optimally have.

Slightly costly approach, having to buy another sword, but I guess that's the only real solution... I guess there's also a pretty good chance it could just lower it by one star if it fails and not blow up... although with my spectacular luck I'm guessing I should get the second sword just in case.

==EDIT==
Oh, wow, I got lucky!

It lowered it by one star and didn't asplode, so now it's at 11 stars... so I'm guessing that means I can use the shield scrolls... too bad I wasted all those clean slates but... meh, suppose it could have been a lot worse.

Thanks for the tip about the star enhancements!