89 in Eng Adv, Math Ext 1, Math Ext 2, Phys and Chem would give around 97.2.
This is also a good reason why school ranks are not that effective. A school could get 500th but every student could have an ATAR above 97, since (unofficial) school ranks are based on the percentage of Band 6s.
Yes, it is feasible. My ATAR was 91. I only got one band 6 in English, which is 93. And the other 8 units that counted towards my ATAR the mark ranges between 83-89.
As long as your subjects scale decently, and your band 5's are high band 5's and very close to band 6, getting over 90 ATAR is achievable.
Yes, it's very possible. I would say works more if you do higher scaling subjects like chem, maths and english advanced. I know people who got low band 6s and mid band 5s and end up with a 95+ atar. It's possible, but don't rely on scaling as it changes each year.