Regarding the al a carte restaurants, in addition to the three reservations you can make in advance, they will accept "day of" reservations after 4pm at the customer service stand in the lobby. That is how we handled it, as we found the dinner buffet to be awesome and felt tied down by a set reservation in advance. We did the Japanese (good but super-salty!) and the Mexican (Loved it!) and the rest of the nights did the buffet. There are tons of amazing restaurants in Playa too. The beach sidembuffet is good for an early dinner too.
We tipped in dollars the entire time we were there, and the people were happy to receive them. Next time we'll do pesos,