Yes, yon is yarn over needle.
This is a double increase. The p1, yon and k1 tbl are all worked into a single stitch. Bring the yarn to the front to purl one but don’t take the stitch off the left needle, then take the yarn over the right needle to the back and knit into the back of the same stitch on the left needle. Now the stitch can come off the left needle.
Here’s a video for k1, yon, k1 worked into a single stitch. Not quite what your pattern is calling for but similarly worked into a single stitch.
This increase sounds more complicated than it really is. It’s really pretty easy to work and very effective. The yarn over is a neater less bulky way of working the double decrease.
Is it an increase, or a decrease? You say increase twice, and you also say decrease a couple of times. It should be an increase, right? Or am I missing something?