To get more accurate matching at the seam lines, try going to the ironing board and placing a pin exactly through both pieces at the 1/4" seamline. The pin should be straight up and down, into the soft portion of the ironing board. Then place a *tiny* dot of Elmer's white washable school glue inside the seam allowance and press. The glue acts as a "pin" that doesn't shift when you sew. Afterwards you can peel the seam allowances apart where the glue is (moisten with a fingertip of water if it doesn't peel apart easily -- usually caused by using too much glue).
For the center of your block (last photo), you want to "swirl" the seam allowances so they aren't all piled up on top of each other. To do this you usually need to release a stitch or two. I couldn't find a tutorial that shows how to do it with the number of center seams you have (although I'm sure there are some on the net). This one shows you how to do it with a four-patch:
http://joanne-everyonedeservesaquilt...ing-seams.html
With a block such as yours, the "swirling" ends up looking like a small flower.
Edit: Just wanted to mention that your center points are not going to be exact. This is because the cutting and piecing of the individual segments (as in photo 1 above) have resulted in triangles that are not completely similar. If you were using strip sets to cut the triangle segments, check to make sure that (1) all strips are cut evenly, (2) your seam allowances are sewn evenly, (3) measure completed strip set to see if all strips are even and measurements the same throughout the strip after pressing, (4) check the accuracy of your cutting of the strip sets into triangle segments. Inaccuracy with any one or more of these steps will result in uneven triangle segments as shown in your first photo. The inaccuracies show up most in the small center triangle portion of each segment.
Edit2: Guess I am long-winded today! Just wanted to add not to give up at this point. Many small inaccuracies are not noticed at all in a finished quilt. A quilt does not need to be perfect in order to be a great quilt!