Hello Laura,
The 99K won't do reverse. You don't need it. Just turn the work with the foot up and needle down and sew back the way you came.
If you go to my blog (link below) you will find a heading at the side for posts about stitch length regulators. Everything you need to know is explained with photographs.
The 99K is an excellent machine. This doll quilt was quilted on my 1927 99K hand machine, which serves me very well. It doesn't even need a fancy quilting foot, just an ordinary hinged regular foot, and jam the presser bar down to maximum pressure.
Your machine could end up as being your favourite. You just need to get to know it.