You 100% definitely still want to flush when you're using organic nutrients otherwise you'll have the same kind of problems that synthetic growers get when they don't flush
The whole point of living soil is to give your plant certain nutrients at certain intervals, releasing over time, not all at once.
So yeah if you randomly add concentrated synthetic liquid nutrient to your plant you're going to kill it
One easy way to tell if that guy's bs is if you read his...
Yeah bro you definitely want some sort of air intake, may need to get a bigger opening if your exhaust fan is causing negative pressure inside the tent
Venting into the same room won't hurt anything as long as it's big enough in which 250sq ft would be fine