Many points to think about hereā¦
Nikon DSLR is complicated, because there are many things which can be adjusted and/or preset about the pictures, and even more features which (some) people want to adjustable in their cameras, like what each dial and button does. My Nikons must have hundreds of adjustable things in the menu, only maybe a dozen are really important and need to set mostly just once.
Is somebody asks for 300dpi resolution, he/she does not know what he/she is talking about. DPI setting is just a flag in the file which can be changed at will without altering the content of the file at all. Does changing the font size from 10 points to 12 change the content of a Word file? It does not, same thing with the dpi setting. The only thing which matters is the pixel resolution, they should ask for a certain amount of pixels, like at least 16 Mpix etc, not DPIs. Which is sharper and higher resolution, 4 Mpix file at 300 dpi or a 36 Mpix file with 72 dpi?
Then: if the first picture came out too dark, why change the lens? Increase exposure. Either by using exposure compensation with auto exposure, or using manual exposure and checking the histogram.
Lens should not affect the white balance, that must have been tweaked while changing lenses. Use either auto white balance, or choose fluorescent, or 4100 K. There are adjustments for this because it is needed.
Shooting RAW would make this white balance problem disappear, as you could adjust the WB perfectly when converting the RAW file to JPEG. There is never an excuse not to use RAW.
Is having a 12 Mpix file a problem? The client asked for 300 dpi which is easy to set in photoshop for any file, but they did not specify what the real resolution of the file in pixels must be. You could, just for fun (?), down sample your picture to 1 Mpix with 300 dpi and if they complain say it is 300 dpi like they wanted.