No, neither will work and they may even be damaged.
12 + 1a - 1a + 1 = 1 + 0 + 1 = 2
No. If it works at all, it will be underpowered, and possibly damage the speakers. To power 12V speakers, you have to have an adapter with EXACTLY 12V (no more, no less), and at least 1A. You could hook up a 12V 4A adapter if you wanted to, your speakers would just only draw 1A of power. Make sure you use a 12V adapter though!!
Output: 5V 1A
Yes.
Unfortunately no, if the device calls for 2000ma you will need a 2A (amp) power supply to adequately power it.
No, the new adapter will not be able to safely supply enough current.
1a.
(x + 4) + (x + 5) = 2x + 9
Yes the power pack can supply up to 1 A, so 500 mA or 0.5 A is within that limit.
1a + 1b because they are different terms and could not be added together. But 1a × 1b would be 1
1a = 1(52)= 52