I have a Half Dome 2 plus tent. I think it's the 2010 model but I bought it used from a boy scout troop that was finished with it. It is in great shape BUT it leaked on my last campout. It didn't seem to be just the seams - it overall soaked through from the fly.
I am prepared to seam seal the tent since I know that's a normal thing to do. However, I don't know where to start since it seems like the whole fly has a damaged waterproof coating.
Suggestions would be appreciated.
Before you seam seal the tent, I would reapply the DWR (durable water repellency) finish of the tent. A tent of that age, it is likely that the DWR is worn off - which happens naturally over time - and that will allow the nylon to eventually soak through.
Here is an REI expert advice article on care, cleaning and waterproofing your tent.