Hey odd ball... and the below is just my opinion. You might want to double check it with the textbooks, as i might make mistakes too, that you have already have the answers are in there.
1)An airplane is descending with a constant IAS and zero thrust. What happens to the descent gradient when the OAT increases?
Please revisit the lift equation.
Constant IAS - constant Q factor.
OAT increase - increase/decrease what variable of the lift equations
what needs to be compensated to maintain the same Q factor.
FYI you might want to revisit the chapter on drag, if you are maintaining the same IAS drag remains the same...
it should bring you to the conclusion that the gradient increases.
2) An airplane is descending with a constant IAS and zero thrust. What happens to the descent gradient when the airplane reaches a lower altitude?
in this case the steps to analysis this is the same as the first question, however you do not compare/consider OAT as a factor at a instantaneous point. you compare the conditions at different altitude.
it should bring you to the conclusion that the gradient decreases, as with a down drift.
3)An airplane is descending with a constant IAS. What happens to the descent path gradient when tailwind is increasing?
I believe you want to revisit part in the book defining gradient (air/ground). air gradient is independent of wind.
tailwind decreases the ground gradient and and actual flight path angle will reduce accordingly.