This will work. Experience from my J/30 Rhapsody recore blog and this old forum post.

My questions:
- Should I listen to this guy? Is solid fiberglass the way to go? - no - properly isolated core is stronger and lighter
- If coring, then is my cedar plan viable? I could order some core materials, but it would be pricey. - your cedar approach looks good. You may save yourself some time if you "glued" it to a mesh fabric screen off the boat, then use the sections cut to size and place in the area to recore.
- After reading other posts about transom repairs it seems that the exhaust is a problem area. This area seems solid on Quicksilver, but should I address this potential problem are while I am at it? If so, what is the best way to tackle this? Remove the exhaust tube and make the entire area around it solid thickened epoxy with a new tube glassed in to isolate from the core.
- Should I add some blocks to mount a future swim ladder? Yes! I did the same using solid glass so there could be no leaks into the core.
- What are this forum's recommended thru hulls? I was able to salvage the original valves. My J/109 has Marelon valves which will never corrode and if I did this again would size for the appropriate valves.

ps - You are doing it the way I should have done it from the outside. I thought I would save work doing from the inside, but it was harder because of the access,