ALT world & Mega Pencil - what does my money get me?

I've been playing Pokemon Picross for a little while, and enjoying it, but I've started to hit the pay wall now. The game doesn't seem to be very good at explaining what some of the higher priced unlocks will actually get you, and I'd like to make an informed decision before spending any real life money!

ALT world is 800 picrites: I've done the ALT world tests, so know that it unlocks the slightly different types of puzzles, but how many puzzles do you get, and are there additional unlocks where you have to shell out more? (like the standard stages unlock)

Mega Pencil is 500 picrites: Is this to play the one stage, or once I've purchased this can I play any of the Mega pencil stages. What is different about this puzzle?

The Nintendo e-store says that if I purchase 5000 picrites you will be able to obtain Picrites for free in this shop, so about £28.78 of real life monies. Does this mean you get unlimited picrites once you hit this, or that they give you a one off bundle of x picrites?

Are there any other unlocks further in the game which require more picrites? (Other than the standard unlocking of the next world & unlocking Pokemon slots)