In my area, if you core the pond you don't have to worry about sealing the rest of the pond. But I do know rolling in clay needs to be done in other areas.
Around here, if you scatter some brome on the island it will soon crowd the weeds out,
Same here on sealing the pond situation unless we run into something unusual, like gravel from an old creek bed, a sandstone outcrop, etc. In our pond at home we had to go down nearly 6' in places to seal the core trench because the dam back was near an old seasonal creek and we had a lot of silted in soil as well as some gravel where the old creek had probably been at some point in time. We finally got there though. Probably would have been faster with the backhoe but we did it with the scraper. There is plenty of compacted clay in there now. On my daughters addition project was afraid of running into the same thing but hit the clay in just a couple feet so just scooped it out with the dozer and put the excavation on the back side of the dam to establish the bottom of the core.
I had not thought about Brome. Fescue is the grass of choice around here and like you said, if it establishes well it will pretty well crowd out the weeds.
