I know little about this but I do have a couple of suggestions.
Perhaps try to cut the lead out from around the top of one boltat the
joint to allow inspection. If it is good, just make a sheet metal dam
and pour in molten lead.
Maybe put in additional bolts. This involves drilling a large diameter
hole deep into the lead and an access hole from the side, putting in a
long threaded rod and putting two nuts with a large washer on the
threaded rod via the access hole. Then you fill the access hole with
some lead and epoxy and pour lead down around the threaded rod. I
believe I got this idea from Practical Sailor who discussed this same
topic once.
What could these green corrosion products possible be? Are the bolts
really SS? Check the tops of the bolts. If they are SS what could
this material be? Look into SS corrosion and then in the CRC Handbook
to see what the possible products look like.