Should I buy a keylogger and hack my cash?
Why not a RAT? You can get Darkcomet or any other RAT and use it to steal the accounts, and if an account has JAG enabled you can manage to get around it with a RAT.
Should I hack RSPS' servers, swapping the client for a FUD JDB?
If you can, why not?! You could get more money.
What's the method of gaining RuneScape GP that's actually worth it?
No idea. I'd say stealing it.
Can you still bot at RuneScape without a too high ban risk?
Seems like you won't get banned if you are P2P IMO.
Is RuneScape worth it?
That depends, what do you want to achieve? Let time pass by, earn some money from it, have a nice game to play, etc.