When rejoining the remaining sts with the sts. I have this yarn exposed on the vertical slot when I work 6 more rows as above **. Is this right?
I'm not exactly sure what you mean by that question. But the additional information did help a lot. I think I understand where you are and what you're doing now anyhow.
Maybe you understand all this and don't need all this but here is what I get out of this.
When you get to this part:
Then it ask me to change into a size 3 needles to K3, yo, work 2 tog, seed 14, work 2 tog, yo seed st 2, turn and continue on these 23 sts only.
you have 67 stitches on the needles. You switch to the #3 needles and knit the 3 edge stitches, then yarn over (I'm guessing this is a buttonhole), work 2tog, seed 14, work 2tog, and yo again (another buttonhole), and seed 2 more. The other 44 stitches on the needle you ignore for now, if fact you could put them on a holder if you wanted, but you don't have to, but be sure not to work them for this next part.
Turn and work in seed stitch (except for the garter edge 3 sts) back across the 23 stitches (the yarn overs kept the stitch count even in spite of the 2 decreases).
You are at the front edge now and you turn and k3 and seed stitch across the other 20. Repeat those last 2 rows until you have done 6 rows. You will end at the edge opposite the k3 edge. Cut the yarn leaving a nice tail.
If you put the 44 stitches on a holder you will now need to put them back on a needle so that you can begin working on the side closest to the place where the 6 rows extend past them. Put the 23 stitches that you worked the 6 rows over onto a spare needle until you catch these other stitches up to the height of the 23.
Hold the sweater right out with the needle with the 44 stitches in your left hand. Starting at the end which adjoins the 6 row extension and taking the yarn so that you can knit, (work 3 together, seed stitch 3) repeat 7 times and then knit the last 2 stitches. You had 44 to begin with you will now have 30.
Next row: turn and seed st across the 30 sts.
repeat the last row 5 more times for a total of 6.
Now you should have a WS row facing you. Seed stitch across the 30 stitches again then take up the needle with the 23 stitches that are being held and hold them so that you don't have any twists or anything and are knitting from the slit side first, over to the front edge. Seed stitch across those 23. Now you have 53 all on one needle and are ready to do whatever it tells you to do next.
Starting that first row over the 44 with a work 3 together makes the slit kind of big on one end but otherwise I don't see anything weird. The slit might look better if it had an overcast stitch or a buttonhole stitch or something worked around it to neaten it up, otherwise it looks like all right. Maybe they tell you something to do to neaten it in the finishing directions.
If none of this helped you carefully explain your question again.