-You should be able to measure some small portion of the core
. My guess it's 1/2".
-As mentioned, exterior ply is fine, same glue as marine
-Poly resin is also fine. Boats have been extensively repaired, including large hull
openings, with that for decades. The boat
in my avatar had the cored hull
opened up for about 10 ft long and 1+ feet wide just above the water line. Core
replaced and ply resin glassed. 23 years later when I saw the boat
and talked with the subsequent owner there were no issues.
-I would build up to go solid/no core under penetrations like the steering pedestal
. You can build up a solid chunk at home if you wish to put under there, then final glass it in. Build it up on wax paper if you do.