Personally I think it’s wonderful that you are about to learn to fly, especially at aged 19. Your experience with two local schools is indicative of the appalling standards that some schools operate to, be prepared for it.
On flight instructor courses I teach two aspects which I claim are unique to my courses, Customer Service and Duty of Care. Some of the respondents to your initial post would do well to dwell on both aspects themselves.
Of course you will pick up bad habits from flying a simulator but so what. Car drivers brake with their right foot only but that habit doesn't prevent a FI teaching them how to use both brakes to control an aircraft. All new students try to taxi steer the aircraft with aileron but again any FI can teach them not to. The natural reaction in a well developed wing drop stall is to pull back when you get a windshield full of ground but again any decent FI can overcome that with proper teaching.
The problem is that too many FI’s (apart from not understanding customer service) can only instruct to ‘fly by numbers’ with a rehearsed script. Teaching is a creative art and every student is a unique product of biological engineering, put those together and you should be able to teach anyone to fly, regardless of their limitations,bad habits etc..
In response to your icing questions
Holdover Times depend on the type of precipitation falling ( if any) after de icing and obviously time. On a few occasions I’ve had to return to stand after 30 minutes due to the temperature and falling snow (as an example of the most limiting time).
I know what the mix of fluid and water and the temperature is because the handling agent gives me a copy of the deicing operation, it’s from this that I can work out the holdover time.
Good luck with your training