So now with WRC+ All live, will we see crews that are waiting before the start of the stage watching live from the cars that started before them?

I'd say most certainly we will. We have already seen this for example in Finland town SS.

Will FIA try to ban it in some way, like they did with in-car splits? Should it be banned?

It certainly changes the "spirit" of rallying, but so does endless recce-videos and WRC+ watching to the point where crews remember the stages. But it's probably hard to to enforce any ban without some excessive measures.