This was in another thread - slowing down the hardening time should be the ticket?
i would not chisel out the good core, having the skin over lapping good core will be stronger.
as for wetting it out, yes wet everything out with epoxy. place the core which is wet on the lower skin which is also wet out, then use some thickened epoxy to fill any gaps, then wet out the top skin and put in place with weight. you also might want to use a slower hardener, as it can sit over night or a full day while you work on the next area. and yes it will squish up some if there is enough epoxy in there. thats why you want to use a large trash bags with sand, you want to have a 100 lbs on the area ( say a 2 to 3 foot area ) in several bags to make sure it get pushed down