At what lvl, if any, do you start 1 hitting coal like iron?

I may be wrong, but I don't believe coal will act like iron at any level.

 

In other words, obtaining the coal will be random, even at level 99 (?).

You'll never one hit coal like iron. It'll stay sort of various. I'm level 75 and sometimes I'll get the coal right away (1-3 hits) and sometimes it takes so long I just click the rock again to reset the algorithm.

You cannot 1 hit coal, unfortunately.

 

However, at very high levels (90s) you mine coal at the fastest speed. That is what I have heard, but it might not be true.