Dremel will be a bit underpowered ... most of the time you have to cut back to solid connected glass/core/glass or glass/glass.
from the photo of the top that probably is going to be a min of 6 in. out from the stanchion base. below I'm guessing 12 in.
7 years ago with Vee Jay at mooring, a valiant 40 came in to his mooring upwind single handed ... missed the pickup and by the time he had got back to the wheel his bow anchor snagged my stb. bottom life line ... so no problem he backed off, rupturing the lifeline and pulling the base completely out of the deck. That required a semicircle patch radius 16 inches from the base as the top skin had pulled clear of the core. and also required replacement of those sections of toe rail which also had to be removed to get to solid hull. here it is looking like your bottom deck skin my have pulled away more than visible in photo.
yes I am thinking that the dark green fiberglass layer you are seeing is a part of the deck hull joint.
send your problem photos as attachment to Vic@VicFarmer.com and I will look at them in photoshop.
