I pretty much finished filling the core void. The rot didn't extend too far, at worst it was around 5 inches in one corner. I did quite a sloppy job I'm afraid and got some epoxy rain down on me but I guess that's how you learn. It's all filled up though and I hope that the repair will last. I am rushing before the rainy season settles in and I only have week-ends to work on it.
I guess my concern was less with the liner void and more with the big cloth that is not so well laminated and kind of dry. Also, is there a way to know what composes layers of the laminate (roving, chopped fibers etc...) by just looking at it?