Simple Bluff said:
To answer you questions:
1. The game is the Final Mix version of Kingdom Hearts that was released back in 2002 on the PS2 only in Japan, so for American and European gamers it will be newer since Final Mix is different from Kingdom Hearts 1 in ways. For one many of the Dalmatian puppies had their locations changed, the treasure chests were changed so that they don't have common gummi ship parts; Gummi Ship missions were added in; most Heartless have had their pallets swapped to different colors; new synthesis items and changes to synthesis in general; 10 new Heartless were added into the game along with new rods and shields, and 2 new Keyblades (which kinda suck honestly

); they rebalanced abilities and changed their AP requirements, some being lowered, while others were raised; changed the stats of certain Keyblades; added in new abilities and the ability to skip scenes; two new difficulties were added and the Heartless do more damage; Deep Dive secret ending was added. Oh and they moved the Red Trinity in Oogie's Manor to being down below so then you can still get it even if you destroy Oogie and finish Halloween Town.
Those are pretty much the changes in the Final Mix game that are kept in Final Mix HD. Only things changed were the camera being controlled by the right stick now, triangle is used to open boxes and talk to people, Summon has been moved to the bottom of the Command Menu
2. Kingdom Hearts 2 Final Mix Plus will be in the 2.5 HD ReMix that will be coming out next. It is hinted to contain Kingdom Hearts 2 Final Mix Plus, Birth by Sleep (Possibly the Final Mix), and they might do another cutscene style thing for Re: Coded, which I would love because Re: Coded doesn't need to be remade to be playable on consoles...>.>
3. All the hidden bosses- Sephiroth, Ice Titan, Phantom, Kurt Zisa, and the Unknown (Final Mix exclusive), will give you trophies, all of them are Bronze.
Note that the Phantom received a pallet swap, but Kurt Zisa did not.