FYI...use acetone to break 5200 apart, put it in a water bottle and spray it along edges to break it down and slowly use a scraper to get under it and it will work apart over time. About the only chemical I know that will break it apart even after dried. 5200 is only needed for thru hull fittings, if your just sealing the light to thre hull 4200 is enough holding power as long as any hole for a wire through the hull uses 5200.
I would recommend the gelcote repair kits, they aren't too expensive, barrier coat is a good idea but if the paint around the area wears down water could seep in down the road, only complete fix is the repair kit. Covering that with barrier cote might be a good idea as well as opposed to putting the light directly on gel cote, might also make it easier to get off down the road if needed.