So finally my question is; if Honda does find the AC gas leak at the rear evap core, is it really the evap core itself is leaking AC gas or would it likely be just worn out o-rings?
Any single component in the entire system that holds refrigerant is a possible suspect. It can be very difficult to isolate which one is the problem without the right tools. The dye as you describe is intended to be used with an ultraviolet light, when you shine one on a leak spot with dyed oil it will show up a bright green color.
My next question; if it really is the rear evap core that's leaking and needs to be replaced, do you recommend the Honda oem rear evap core that'll set me back about $1,000 (not including labor) or can I use an after market rear evap core instead. Amazon has a seller selling a compatible one about $100 after shipping. I know Honda original is always better, is this something I need to pay $1,000 for?
Under no circumstances should you be buying Honda parts off of Amazon. It's been beaten to death in other threads that there are generic/clones of genuine Honda parts being sold with the Honda sticker. Amazon has no real incentive to root out these bad sellers, either way they make money on a sale. If you don't want to source locally from a dealer, a lot of them have their own online sites, search Majestic Honda, Bernardi Parts, or HondaPartsNow.com, they are all legitimate Honda retailers.