You've already read that skin to skin means oral to genital, genital to genital, or genital to anal skin.
It doesn't mean any other body part. Arms, legs, chest, back - whatever, it doesn't include those parts. It only includes the thin mucous membrane skin.
There is no risk here. None at all. You don't need to test. You don't need to worry. There are no what-if scenarios you can come back with that will change this.
I don't know what is on your abdomen, but it's not syphilis. Test if you need to ease your mind, but it will be negative.
In Oct, you told me you'd do counseling. Did that happen?
Also, no judgement here at all, but if cheating on your spouse causes you such anxiety, maybe stop doing that. I don't know your reasons for doing it, but work things out with your partner. Maybe try marriage counseling as well as individual counseling for you. I don't know if you are going outside the marriage for the thrill, because something is lacking in your marriage, or simply because you can, but it's not working for you. I say this only because your mental health is as important as your physical health.