Re: washing machine filling
Okay, if the main water in is isolated and not on to the machine then the water has to come from somewhere.
The only other place it can come from is the drain through backfilling.
But… the water would not then rise above the level o the drain, gravity and all that comes into play making that impossible.
And, likewise, if it was getting water from the mains supply even were it on then it shouldn’t fill above the drain either.
So it doesn’t really matter how slice it, either what you describe should be not possible to happen, something is very wrong with the installation or, there’s something wrong with the plumbing.
K.