I am not all surprised by the walker issue- most people/professional have no clue. And in the pre-joint classes, they often just way "get a walker". There are pros/cons to all styles with the swivel ones are by far the worst due to many safety and instability issues- I would never recommend one. I ended up using crutches (for stairs) and front wheeled walker at home, and eventually a straight cane.
Regarding the ROM: 145 degrees of knee flexion- that is no where near the norm; norm values for knee flexion (while on your back)- is 130-135 degrees and the numbers depends on several factors including scar tightness, quad tightness, having a larger calf (like a very muscle bound person), or a large calf, etc. When you measure knee bending (flexion) in supine, it is often less due to tightness of the quad; in sitting- this is the easier position to cheat and have a inaccurate measurement and 145 degrees is possible to measure- but it's not due to pure knee motion (usually comes from hip motion) and often sloppy technique by the therapist.
As far as how much you need? Many patients get hung up on the number- for example, a quest to get to 125 degrees for example. For functional purposes (this is for daily activities), usually a minimum of 90 degrees is needed (like to go up steps) but around 110 bend is optimal for the minimum number. With bending, If you get more great, but having more won't necessarily change your function. In comparison, if one has a lot of scar tissue and can't bend past 80 degrees (while on their back, for example), this has a much larger impact on the ability to do functional activities.
There's a need to get full extension- which is often lacking due to tightness as well. A lot of people forget the value of having near full extension (often called zero, 0 degrees) which is needed for optimal function.