We're at the same point post-surgery. Is your brace adjustable? You may try walking with it set at 30 degrees....that is pretty safe, as the brace will catch you if you fall. Then practice squatting up and down (standing) within that range. Also for strength and to get the muscle nerves fired up again, do the isometric quad flexes, lying without the brace.
I've been ok with stairs from day 1, as long as I take them one at a time with the leg in the brace.
Last night I felt comfortable enough going from bed to bathroom without the brace, but I don't want to tell anyone to do this until you are really sure. I have had no buckling problems, but do tend to keep the leg mostly straight when not in the brace, so I don't know if "walking" is the right term. I have had really no pain from day one, so I am not sure where yours is coming from....but I think that is normal. (?)
Your ROM is great! It souinds like you just need to work on strength, and as I said in my previous note, I think PTs can be really conservative about pushing strength work this early.
Just as a reminder to all, I had this same thing happen to my left leg 2 years ago. Now, 2 years later, it is about 99% normal, so hang in there!!!