Integral non linearity in ADC
No, a +/- 1 LSB error is not a percentage value except in the special case of an unusual ADC with a total range of 100 steps.

Most (virtually all) ADCs have binary outputs so an n-bit ADC can encode the input as 2n steps, so an LSB is 1/(2n) or 2-n of the full scale output.

So a 1 LSB error is 1/2N of the full scale output or (100 / 2N percent) of the full scale output.

EDIT : you are thinking along the right lines. If your 8-bit ADC has a +/-1LSB INL error that means your readings will be within +/-19mv of the actual value.

Your first table makes another point : even in a perfect ADC there is still an error due to quantization : the process of rounding a real value to the nearest integer equivalent. This introduces an error of up to 0.5 LSB hence the additional 1/2 in your table.

Your third comment is simple to understand : 4 LSB-sized steps of a 12-bit ADC are the same as 1 LSB of a 10-bit ADC.
Or put another way, 4 * 1/(2^12) = 1 * 1/(2^10)