I started a pair of these over a year ago and am a little past where you are in the pattern.
Knowing me, since I haven't done them by now I probably never will.
Chart B shows where the increases are. Every time the chart jogs out one square beyond the row below it, that is where you put an increase. Originally you were working over 27 sts and then doing the purl "seam" st, then worked the 27 again and another "seam" stitch, for a total of 56 sts. That is what you have when you begin Chart B.
Row 1 of Chart B doesn't inc, but you work like you have been doing.
Row 2 of the chart is the first inc round. Work across the first 27 sts reading the chart from right to left (like usual), you will now have the "seam" stitch as the next st on the left hand needle. Make the inc in the strand between needles just before that "seam" st, purl the seam st in MC, make the next inc in the strand between needles right there, then work row 2 of the chart from left to right. (Note: if your dividing place for your ML seems to be in a bad place for increasing right there, move some stitches to make it easier.)
Work two rounds even and then do the next incs in the same spot, just before and after the "seam" stitch.
Repeat this until you have increased 18 sts and have 75 sts in total.
On the gap issue.... When you do the set-up row, when it says "pick up and k3 stitches across gap", if there seems to me more room in there than that you can pick up a couple more stitches there. Then on the next round work 2tog as you need to to get back to the right number of stitches. (Or possibly leave them if it wasn't much of a change and your thumb is big enough
) Another thing that helps sometimes is when you come to the "corner" stitches where the gap is on each side, to work the first stitch through the back loop to twist it and close any hole. Last but not least if there is still a hole, sew it together with 1 or 2 well placed stitches.