Hello!  Welcome to Omnimaga.

You shouldn't worry about people stealing your program.  If somebody does do that, then the community tends to get VERY angry at them (and shun them, and ban them, and dislike them in every way...  Of course, looking at other people's code, and sharing ideas are highly encouraged.  The calculator community tends to be very open-source).  Of course, accusations of theft are equally disliked, which got a few people in trouble a while back when they got too paranoid.

1) If you don't like the idea of having to store x and y coordinates into two vars, you could just use modulus to combine them into one.  If I do this:
335->A
Then I could extract the x-intercept by doing 335^255 (which is 335 mod 255, which returns 80) and the y-intercept by doing 335/255 (which returns 1, because there are no decimals in Axe.)
So basically, to store coordinates into one number, X-intercept+(255+Y-intercept)->variable.

2) To do sound, use the Freq( command.  I don't know the exact numbers, but try experimenting with Freq(1250,(add any multiple of 1250 here)).  This should get you a sound that is relatively low.  (Of course, you need 2.5mm headphones to listen to sound with the calc...)

3a)  First, you don't really need decimals at all.  If I wanted to use decimals, I could just multiply the number by 100 (or better yet, 255), manipulate it, then divide it by 100 (or whatever you're inflating it with) when displaying on the screen (because a 64x96 screen is pretty inaccurate to begin with)
That being said, I'm not entirely certain how I would make a project follow a parabolic path, but I would try adding gravity, simulating how much power is in the projectile, and simulate real life.  You should probably ask Builderboy.  He's good with physics.

3b) Inflate all the numbers (let's pick an inflation number of 255, because it's cool), use the standard y=mx+b slope formula, find out what the next number is, then divide by 255 (or the inflation number) again.  Then use Pt-On (or whatever similar command)

4) Number^2
If this returns 1, it's odd.  If it returns 0, it's even.  (or perhaps its the other way around, but you should be able to figure it out)

Edit: Apologies if I didn't make much sense, or made mistakes - I'm a bit sleepy.

Which can be found over the divide button by pressing [2ND].

Oh!  User input could be the tunnel game!  To set the first number (a) I would need in the first section to fly around trying to collect the exact number of coins (so if a = 5, I would need to collect exactly 5 coins). To set b, I could do triangles, and c could be squares...  And a black circle will subtract everything by negative two...

Hmm... I should make a quadratic solver in Axe that get more accurate when you mash the buttons while it's crunching the numbers...
I should also try incorporating a tunnel game somehow too...

After owning and playing both Scribblenauts and Super Scribblenauts, I can probably say that any port of Scribblenauts would take up a huge bank of memory just for the dictionary.  If one were to remove a bunch of usually unused words, and simplified the interaction between objects so that all objects could be described by a small set of characteristics, it might just be barely possible.  Maybe.

And you're not a newbie 

Ah, I see.
I had assumed that the two were related to each other, so that signing the application would also handle the expiration count.
So if I were to sign the app and transfer it to another calculator, would it still have the expiration count?

Oh ho!, good sir - this shall be teletranscombulated to my calculator with rapid speed for prompt testing!

Edit: And, alas, it did not work for me.  I created a short program that would display some text then wait for a key press to quit.  I tested it to ensure it would operate normally, used Ducksign, then tried using it for about 16 times, but it defragmented.  Could the fact that I used Axe have anything to do with it?