Let us try as your professor suggested as lim R->0 the equation becomes 0/0 which is undetrmied so let's use L'Hospital's Rule. we get $I(t)=\frac{V_b }L .t e^{-tR/L}=\frac{V_b.t}L$. Yes $$I(t)=\frac{V_b }L .t e^{-tR/L}=\frac{V_b.t}L \, .$$ Yes because an ideal inductor is short circuit for steady state. as you might know inductive reactensereactance $X_L=j.\omega.L$$X_L=j \omega L$ as it's DC, frequenciefrequency is zero it's practically a short circuit .sincesince it violates kirchhoff'sKirchhoff's voltage law hence a large current flows in the circuit .