Some games will have cheat codes, but it varies. Some might also have various save files available online that allow you to skip sections. But there's no full-proof way to avoid playing the game if you're looking to hard-rip.
There are other ways like tile-ripping, but that can get tough. It's a lot of fiddling with tile editors and piecing things together.
The reason there's no "true" palette is because while on modern computer we use 24-bit colours (if you ignore alpha), the NES was restricted and didn't follow the same rules. This is similar to a previous discussion about the Game Boy colours. The GB's palette was basically grey and had 4 levels of greyness: 0, 1, 2, and 3. It didn't use 24-bit RGB, so there aren't any values in that format to grab. Mathematically, we could split the 24-bit colour range into 4 equally spaced parts:
#000000
#555555
#AAAAAA
#FFFFFF
However, that's assuming that the Game Boy's black was the same as #000000, and that its white was equal to #FFFFFF. Then we also have the can of worms dealing with how the screen was actually green-tinted, and whether you would consider that more "true" of a palette than simply grey.
The same kind of goes for the NES. The fact of the matter is that people who make the emulators have simply had to do what they deem is the best approximation of converting NES colours to 24-bit RGB, and unfortunately that isn't always consistent.
All that being said, this Wikipedia article seems to get about as on-the-nose as you can get with this, so it's probably the best you'll find.
As for keeping palettes consistent for your uses, I feel like it'd be easier to simply replace the palettes of sprites manually to match whatever you want them to, rather than re-ripping them. In GIMP you can do this using the colour select, or even just by converting the image mode to indexed with a custom palette. I'm not sure what the equivalent would be in other programs but it's likely something similar.
There are other ways like tile-ripping, but that can get tough. It's a lot of fiddling with tile editors and piecing things together.
The reason there's no "true" palette is because while on modern computer we use 24-bit colours (if you ignore alpha), the NES was restricted and didn't follow the same rules. This is similar to a previous discussion about the Game Boy colours. The GB's palette was basically grey and had 4 levels of greyness: 0, 1, 2, and 3. It didn't use 24-bit RGB, so there aren't any values in that format to grab. Mathematically, we could split the 24-bit colour range into 4 equally spaced parts:
#000000
#555555
#AAAAAA
#FFFFFF
However, that's assuming that the Game Boy's black was the same as #000000, and that its white was equal to #FFFFFF. Then we also have the can of worms dealing with how the screen was actually green-tinted, and whether you would consider that more "true" of a palette than simply grey.
The same kind of goes for the NES. The fact of the matter is that people who make the emulators have simply had to do what they deem is the best approximation of converting NES colours to 24-bit RGB, and unfortunately that isn't always consistent.
All that being said, this Wikipedia article seems to get about as on-the-nose as you can get with this, so it's probably the best you'll find.
As for keeping palettes consistent for your uses, I feel like it'd be easier to simply replace the palettes of sprites manually to match whatever you want them to, rather than re-ripping them. In GIMP you can do this using the colour select, or even just by converting the image mode to indexed with a custom palette. I'm not sure what the equivalent would be in other programs but it's likely something similar.