Is Motor KT Linear with current?
Is kt constant (torque relationship linear) with current? 

If it is an air core machine then yes, Kt will stay constant (and thus torque will have a linear relationship to current) as you will not saturate air (but equally kt will be low)

For machines using some form of permeable material (iron...) Then no it is not as the B-H curve is not linear and thus kt is not constant.

However... It comes downto the actual machine design. Was it designed to work relatively deep into saturation and as such the difference between 10% and 90% current is measurable then further current will show a significant drop in incremental torque.

Was the machine a 'lazy design' and it operates around the origin (and thus 10% and 90% are very similar...) ... The decrease on kt due to more current might be negligible enough that you can still consider it constant (within tolerances)