Is air in the toilet isolated, to prevent seeping into the cabin?
If someone can get chemicals on board a flight, they can also get a bomb on. Bombs are easier to make and more effective. Better air-conditioning won't help here, better security will.
It is possible to isolate the toilet... but the problem apparently wasn't just inside the toilet. The toilet isn't pressure sealed, so some smell will escape regardless. It's impossible to completely seal the toilet from the rest of the aircraft, and could be potentially dangerous if it was
The aircraft does introduce fresh air into the cabin, but the compressors to do so only have a fixed capacity. The capacity of these could be increased, but at the price of extra weight, fuel consumption and cost, it's not worthwhile for the 1 time in 100 million that a flight has to turn around for this reason. Even with higher capacities, it's not possible to just constantly flush the cabin air - and remember that all that air needs to be compressed (to maintain pressure) heated, and filtered. All of that costs more money, and adds more complexity, weight etc.