Remember, you are only EIGHTEEN DAYS into a recovery that takes, on average, THREE HUNDRED AND SIXTY-FIVE DAYS! Most people feel better around three months, but it takes that year to fully heal. Heavy PT, and forcing your knee to go where it's not ready to go, will only cause your recovery to go even slower with many setbacks. Spend as much time as you can icing, elevating and letting your knee recover from the assault that is a TKR, in between walking, gentle stretches and movements.
You own that knee; not the PT, nor your doctor, nor his evil PA. Therefore you, and you alone, make every single decision as to how you want it to recover. IMHO, I'd tell that PA where she can put her 90 degree deadline, and tell her she's fired. You also don't need PT, but if you decide to continue PT, you can choose who to go to and what they can and cannot do. Remember, ultimately, it's you paying for PT, therefore you're their boss, and you tell them what they can or cannot do-they have no ability to do any more than you allow. If they are allowed an inch, they'll take a mile. You are in charge and are the only one who decides the path you will take.