8. Is there some parameter to decide the speed in which one should start the vilambit gat..

Is there some parameter to decide the speed in which one should start the vilambit gat, especially after playing the jod, ulta-jhala and some Ustad Vilayat Khan style taans? I understand that earlier in Ustad Enayet Khan's time the pace was a bit faster. Has the gayaki ang influenced the pace of vilambit gat to be slower?

Off hand, I can think of two reasons:
a. Great improvement in the quality of sitar, enabling greater resonance and ability to sustain greater gaps between phrases, and therefore slower tempo.
b. Increasing influence of khayal which reduced the tempo of execution of vilambit khayal, and similarly also influencing the playing of vilambit gat on sitar. Further, there are no set rules relating to the speed at which vilambit gat should be played. It would, in my opinion, be governed by the speed at which a player is able to execute chaugun taans.