That is not a stupid question. A YO is only moving the yarn over the needle, and what type of stitch should come after it will be written in the pattern. The reason videos show a stitch afterward, is so that the YO is secured and you can see what your work will look like.

Whether or nto a cable is reversible depends entirely on that specific cable. If you are talking about a simple left or right twist cable, you could knit the sleeve top down instead of bottom up with no problem. I suggest you knit a small swatch with the cable and look at it as you turn it and see if it works.

Traditional blocking won't do a thing for acrylic. If it does "need something", this is what I advise: Lightly spray the scarf and pin it (this will need to be done in sections since it is an eternity scarf) to an ironing board. Place a clean cotton dish towel over the section and hold a steam iron over the towel very close to the towel, but not touching it. And be VERY careful not to touch the acrylic yarn anywhere, as it will immediately melt. Slowly move the iron around the pinned section for a few seconds, then remove it and let that section completely dry before moving the scarf and pinning another section. Alternatively, you can actually put the iron directly onto the towel and just iron the piece. It will permanently set the scarf in the position you've pinned it, so be very sure you pin it as you want it. This is known as "killing" the yarn, because as I said, it will never go back to it's original (often lumpy) shape. It often also softens the yarn and the piece is then usually a bit limp. Depending on the piece, this may or may not be desirable.

Hayjud, I just recently started a pair of socks for my husband, toe-up on two circular needles. I think they're great, because there is no grafting at the toe. Also, if you have a limited amount of yarn, and you're not sure if you'll have enough to finish a pair, when you split the yarn in half and do toe-up, you're done when you run out of yarn. My new favorite thing to do.