You are quite correct Wiz
Tailwheel
CASA always works to the type data certificate. They have provided a number of operators with an exemption against the 1:36 reg, but it goes on the requirements of the TDC, not the seats fitted. So, as Wiz has experienced, if your aircraft is rated for 160 seats and you chose to operate with 150 seats fitted you still need 4 cabin attendants. The only way to get around this is if the certification process included a demo with 150 seats fitted using 3 attendants, but this is rare as manufacturers are keen to get the max number of seats certified.
An option is to provide CASA with a full evac demo (rather that a partial) to prove that 150 seats fitted in an aircraft certified for 160 works (given cabin layour etc). Last I knew CASA was reluctant to do this as they don't want to get into the business of Type Certification, unless an aircraft is built in this country of course.
Gen Anaesthetic
Laudable motives indeed and great to see someone genuinely concerned that this is not a retrograde safety step. I guess we can only look to the most recent cases to see if 1:50 works. The ditching of an A320 into the Hudson is probably one of the more recent. 1:50 crew compliment and all evacuated safely, despite a passenger opening a rear door when thay should not have (otherwise the aircraft would still be afloat today). Not a fire senario as your concerns raise, however most fire senarios in our industry are sadly a result of a catastrophic failure for which even a ratio of 1:2 would not ensure a positive outcome.
This will remain an emotive issue, particularly with unions involved. Just as we are seeing in relation to the Qantas maintenance issue - a union 5hitting in their own nest to further their cause. CASA's change to 1:50 is coming. Just ask the current Director of safety at CASA who is clearly set on our regulatory environment in Australia becoming aligned to the rest of the world.