Kakashi who is not an uchiha and has no senju dna was able to kamui the arm of the gedo mazo before being summoned by madara
Summoning jutsu is a s/t technique which means kamui is either instant or really close to instant, Sasuke and Itachi are not sensors which means they dont know when Kamui is gonna happen, so before they blink, their heads are gone
Madara is slower than Obito so.....
Your point about kakashi being what? He can't use kamui as quick? He can't use it as well for sure, but the quickness is equal to Obito's kamui since it was stated by madara that adding Kakashi's kamui to Obito's doubled the warp speed.
Yes summoning is a space/time technique and it is really fast. However the mazo had to be fully ripped out of Obito before it could teleport, Kakashi used the time while it was coming out of Obito to kamui it, which is definitely ample time for kamui to warp it.
If Obito looks at Itachi's head than he will be hit with tsukyomi, he would have to snipe Itachi somewhere else.
Itachi and Sasuke may not know when it is going to happen, but they can assume that it will always be a possibillity, and also Sasuke with his new rinnegan ability is fast enough to not get hit by a ranged kamui.
Madara isn't slower than kamui, he could stop kamui from warping Obito which means that Obito can't warp Madara before he stops him from warping. Only a direct kamui with both eyes would be faster than Madara.