Didn't read the long post with no breaks, too difficult for my brain, so excuse me if this is repeating.
First cake ideas:
A pirate ship. I believe you can find a mold that will give you a great stand up pirate ship. Minimal to no decorating needed.
A pirate. Wilton makes a pirate mold. Cake by numbers.
When DS#1 had his pirate party when he turned 6, I did a pirate map cake. Sheet cake. Decorated it with smooth butter cream icing, tan colored. I used a toothpick to draw a line. I used the glittery/glossy blue tube gel frosting and filled it in; make very realistic looking water. I added a few small gummy fish. I put brown sugar over the top of the rest of the cake to mimic sand. I then made trees out of pretzels and green frosting. I can't remember exactly how I did it, but you can find directions online. I then put some rock candy for rocks in various spots. I used frosting to draw an X and put a cake topper decoration of treasure chest by it. I then used chocolate coins around the sides to decorate (also frosted with the tan frosting). My camera went so I didn't get any pics, but I got lots of ooos and ahhhs from both parents and kids.
Treasure maps: Oriental trading sells really cute pirate map paper that you could put in the printer. You can also use parchment paper or tan printing paper. For a neat effect, very careful burn a bit along the edges in a few spots after ripping some rough edges. Roll and seal with a bit of wax, a sticker, or tie with string.
Treasure: We used small treasure boxes (cardboard) from oriental trading. Inside were $100 playing cards, fake coins and dollar bills, money stickers, Mardi gras necklaces, skull rings, chocolate coins, gold bubble gum, candy rings/bracelets/necklaces, treasure chest bubbles, money pencils, money notepads, $100 bouncing balls. It sounds like a lot, and the chests were full, but most of it came from Oriental Trading (coupon & free shipping), clearance sales, or the dollar store in larger lots. Candy of all types is good. Glow sticks (our dollar store has them in abundance) are also always a hit. Sunglasses, beach balls, colored sunscreen, whistles, etc. would be good too. Really, anything they are into is good. You can tie individual treasures in bandanas too. There are endless possibilities.