I have not read the whole thread, but I can tell you my experience from my son's 5th birthday trip. There is a trip report in my signature if you want more specific details on some of the restaurants. We had his birthday noted on every reservation. He did not even make it long each day before taking his button off, so we stopped putting it on (except his actual birthday and he poked himself removing it). At EVERY single character meal we attended, he was given a birthday card signed by all of the characters. This was probably the best perk, because he adored them. While I am sure they are not guaranteed, it would never hurt to mention the birthday if nothing is automatically given. He did get free cupcakes at quite a few restaurants (including breakfasts). We were on the deluxe dining plan anyway, so he never really ate them. A lucky squirrel enjoyed the one at AK that the Donald's Safari breakfast server insisted we take. Side note, never leave food in your stroller.
For my younger son's birthday at
Disneyland, he did not receive anything (no cards, no extra desserts, not even a button-which was my decision as the button poked my older DS). We did an in room celebration, which had a light up birthday necklace and birthday ears that were way too big. He wore the necklace and the reservations were noted. We had a birthday lunch for him with 25 guests and ordered a cake, but we paid for it.
I guess my point is add the celebration to the reservation and your odds of getting birthday magic will increase. By the end of the week, my older DS was like "again." He was actually over it believe it or not. My younger DS was too young to notice either way, so we never mentioned anything about his birthday (other than the party lunch which they already knew because of the cake we bought). Have a great time. Birthdays are magical at Disney no matter what, but a little extra magic is never a bad thing!