The T bracket doesn't do much at all and provides no advantage with the triangular brace there. :)
My machine was not square, so your's likely isn't either. There is nothing there to make it square, nor hold it square if you are able to manually move it square.
Additional holes up the pillar didn't seem necessary. The additional ones on the bottom also work to reinforce the bottom of the frame where the extrusions meet. I was trying to also square all the corners as well as the pillar.