Did they do this with half square triangles? Hmmm... looks like it to me. I've seen tumbling blocks (what the center of this quilt is called, just that this one is a variation). I'll have to try a block or four. If my corners/pieces didn't match, I'd find the true center of the original blocks (make one from scrap fabrics), fold in half, press, and fold again and press...when you open it out..you will have the true center of the original block before triming, and you also have the center of all four sides. Now, open out the scrap block...with a sharpie (black or color you will be able to see through your blocks) draw a line (using a ruler) on all the fold lines. This gives you a test block for lining up your quilt blocks (you could do this on paper, drawing out a block, and penciling in seam lines...but an actual block gives a better referrence). Square up the block. Here is where you probably went off course. Let's say your test block is 9" square...that makes it 4-1/2" from center out...but you want it 8-1/2"...that's 4-1/4" from center out...find the 4-1/4" mark on your ruler...put a piece of painter's tape (or that fancy hot pink stuff) right at that 4-1/4" from the edge of the ruler mark....put the 4-1/4" line on the exact center of the quilt..matching the line you drew to the ruler line you taped off. whatever is sticking out from the edge of the ruler...trim. Repeat for all four corners. There are rulers for this...but if you don't have one...it's not that hard to do. Now your test block is ready to put to work.
Press all your blocks flat (this is where a lot of new quilter's have issues...they don't press every seam...think it's a waste of time..it's a very important step in making a quilt). Once you have them pressed...lay them one at a time onto the scrap block...Make a tiny chalk/erasable pen mark for the exact center, and all four corners. If one side of the quilt block is cut too small, line that edge up to the matching seam line in the test block. Measure in how much you have of the test block sticking out on that side. You want all the other sides of your block trimmed down so they also are 1/4" off (or whatever the amount is).
Yes, this will make the blocks smaller...but they should then fit together. Want the larger quilt size...add a bit to the largest border.
Confusing, but the fix can be done to save the quilt. We won't mention how many times I've done this, lol...happens more than I like to admit.