No one of any level of math skill would mistake the the derivative of x^n as nx + x^n-1. There's no way DFW messed that up, but it's also a pretty bizarre mistake to have a character make if they're supposed to be making mistakes.
In terms of 'meeting the reader where they are,' you would want to telegraph the math mistakes pretty hard, or they would fly past everyone. Making them obvious and dumb is the only way they'll get attributed to the character, rather than the author.
