We have a lot to talk about so I would invite you to ping me on discord (same username as Steemit) or facebook messenger.
Meanwhile, I'll answer your questions.
I have come across various opinions and information about the highest and the lowest possible reputation score. That's why I haven't included that part. 75 and -25 were perceived to be the extremes, and 25 being the middle ground to start at. But @steemsports has 78 reputation. I personally think the code allows any reparation number but achieving it would be hell of a task. Going from 25 to 80 is 10^54 times harder than going from 25 to 26. Each next number, positive or negative, is 10x harder to achieve.
It comes from reward pool which comes from annual inflation. Nothing is deducted from your account except 2 % voting power for a full vote. Read Steemit Reward System 101 for clear understanding.
Changing voting percentage requires 500 SP on Steemit. You can set a lower percentage on eSteem app or you can use Busy.org where 500 SP is not required for vote slider. The slider is inbuilt and appears when you vote.
I have answered your questions but please do contact. I have important discussion in mind regarding your account balances and how you can get a voting slider in just a few seconds.