The slider can reach 0. And on first iteration it goes down from 50 to 25, but up to 76, so a slight asymetrie. Maybe the slider can reach 0, but the correct value cannot.

I have seen a diamond.

Higher. Yes. See below.

With 1 million total bonus you can complete Indira in a few days. And that would "only" require 1000 reset bonus and 1000 warp bonus. You get more reset bonus the deeper you go, but it does not grow thaaat fast. Completed girls give 0.5 0.5 1 1.5 2 2.5 4 6 8 9 11 12 13 14 bonus. While there is a huge boost, if you unlock scenes from girl 7 instead of girl 3, the difference from girls in tab 3 is much smaller. For "speed runs" the fake timers are your limit and for deep runs your money/skills which translates to total bonus.

If you have the diamonds to max warp bonus, do it. If you think about leaving out the last x2 or x4, you can do that too. It does not matter if you up your reset bonus from 1000 to 2000/4000 or get the x2/x4. But inreasing a reset bonus from 5000 to 10000/20000 takes much, much more time, because the girls bonus does not grow the same way as their requirements.