Will wearing skill fortifying items to boost my skill above 100 stop that skill from levelling up?
Solution 1:
No it will not.
Boosts that increase your skills shown level, only increases the benefits that you receive.
You will still level at the same rate that you'd level if you weren't wearing the ring.
Solution 2:
No, it doesn't stop accumulating xp for that skill. I don't see a reason why there should be a penalty for wearing a fortify one-handed ring? Maybe it accumlate a bit slower xp? Most likely the changes in time is more important then the absolute skill lvl. A fortify one-handed just boost the damage not the xp.