I know it is tempting to flood it with epoxy, but your description sounds like the balsa core has lost all integrity. If the core was merely wet, and there was no delamination, I would be more enthusiastic about drying it and a penetrating epoxy. Your description of black mung sounds like the core is mush and has lost all integrity. Part of how the balsa works so well is you may recall the description of "end-grain" when reading about deck cores. The balsa fibers in this direction, under compression, are very strong. Balsa also has good water resistance to rot. However, once the grain, fiber of the wood is gone, there isn't anything left to work with. Pure epoxy is brittle - it needs a core, cloth etc. to have true strength. Also, this little triangle area at the tip of your deck probably should be stronger than you might initially think. I am not engineer, but experience fabricating stuff over the years makes my gut feel that the point where the two sides of the hull and the deck come together and the forestay attaches as a stress area, and it should be fixed for maximum strength.
OK, so where are you now? If you really think the balsa is OK, and you don't have delamination, check out something like Smith's Clear Penetrating Epoxy Sealer™, but I think I would call and talk to their techs and describe what you have before going this route. Their website is:
http://www.smithandcompany.org/
Tap around with the handle end of a screwdriver and listen for sound changes. Sharp rap is OK, dull thud is not. Be aware that on most Ericsons, if not all, the areas under cleats, stanchions, etc., have plywood instead of balsa, and you will get a sound change due to the core change - don't get faked out by this, just pay attention to where you are taping.
It may seem intimidating to take the router/saw to the underside of the deck there, but it really isn't that hard - just a little patience and you'll do fine. Once you cut it open from the inside, you will probably find the bad stuff extends a bit further than you expect. Don't be daunted, dig it out, and give it a good sanding on what is now the underside of your deck skin. You'll be surprised how good it feels to get the goo out. If you read through the link to my mast step recore in my prior post, you'll see how I bedded the new core in a thickened epoxy mix, and then it was just cloth on top of that. Really pretty easy once you start working through it. Also, I talked with West Systems, and with proper temperature control on your resins, work area etc., you can work down to 40F, even upper 30's. Here's a link to a .pdf document on their site on cold work:
http://www.westsystem.com/webpages/userinfo/moreinfo/000-915.pdf
I mention this in case you are worried about running out of time due to weather. I did my recore in December on the Chesapeake with these temps during the day, and colder at night.
Other thoughts you may have already considered. Do remove that wood piece from in front of the V-berths that seperates the anchor rhode locker from the cabin and you'll have much better working space. While your at it, you may want to pull the bow pulpit and rebed it, and fill with thickened epoxy the bolt holes and redrill. Remove all cushions, sails etc. from the cabin before you start this. You will have to clean the cabin of fiberglass dust when this is all done. Finally, go for it. Learning how to work with epoxy and core is a great skill to have if you love sailing and maintaining our classic plastic boats
One last thought, I have mentioned several brands of products - I have no affiliation with any vendor of anything -have just used these things with success.