First part -
I think her 6.5" refers to the unfinished striped HST blocks, even if it says her A and B blocks.
You lose another 1/2" (actual 3/8" because of the angle cut). As long as you're consistent in all your blocks, you should be fine.
The five strips sewn together will result in an unfinished 6.75" block.
If you cut that in half diagonally, and sew it together with another equally sized striped triangle, the result will be a 6.5" unfinished block.
The formula to make HST blocks-
Desired FINISHED size plus 7/8" (1/2" for the seam allowances, and 3/8" for the angles).
So a 6" finished HST block would require you start with 6 7/8" squares.
She's fudging the 1/8". Again, as long as you're consistent, you should be fine.
The next step in the pattern is when she combines the 4 HST blocks into one larger spiderweb block.
She's squaring that to be 11.5", but according to all the other math, it should be 12" FINISHED.
I would suggest you make a sample block with scraps so you can see how it works for you.
Second part -
Without seeing the actual directions and instructions, if you put a 7.5" UNFINISHED plain square on point, the diagonal measurement is 10.58", which will finish at 10".
The formula is the size of the square x 1.41".
Again, she's rounding. And again, consistency in the way YOU make the blocks is key.
I know you wrote 7.5 finished but without the rest of the directions (is it a combo block or already combined?), it's hard to tell what she meant.
eta: Ok, and went and looked at the Half Log Cabin block. Sorry, I don't get her math at all. A finished 7.5" block will be 10.5" on point.
I've never done any of her patterns, and maybe there is a lot of "squaring up," but the math doesn't work for me.