If you have enough, I would definitely make it finish more than 2.5" beyond the quilt top ... but that's me. I like to have 2-3" on each side to quilt on my DSM so what I'd do if quantity of fabric isn't an issue would be to find a place on the flower print where I could match them up nicely & then just cut the excess seam allowance down to 1/2". In theory, sewing the two selvedges together should match up perfectly, but I know it doesn't always work out that way in reality. Having just the one seam would certainly be the easiest option for quilting. I did that for a similarly arranged backing fabric with matryoshka nesting dolls by Robert Kaufman. The backing came out perfectly straight & you couldn't see the seam unless you were within a couple inches of the quilt and were looking for it.
The other option I'd consider if I really wanted those 9" blocks on the back (they sound lovely) would be to add a 2-3" finished strip between the 9" blocks & the print fabric to give you a bit more backing to play with. I'd seam them in with a 1/2" seam and press open.