A1. People use 'full extension' to mean different things. For some it is 0 degrees that you already have; for others it is extension matching your unaffected leg so that you will walk evenly. I found that even a small deficit of a few degrees affects my gait. and I gave up running as it was really a fast limp, but I also had other complications which indicated stopping running, so it's an unknown whether it would have improved over more time.
A2. Whether you can get the deficit back later depends on why you have the deficit. If it is due to lack of quad strength then it is likely to come back as you get stronger. If it is due to shortening of soft tissue then it may also return with stretching. If it is due to structural factors, e.g. positioning of the graft, then it may not come back.
A3. I think you should give your physio a chance. Are you generally progressing? At 5 weeks it is still early days on a 6 month recovery programme.
If your physio is not too worried but you are, have a good talk with them. I would only be concerned if they are not checking your (hyper)extension at every session and working on it as well as giving you things to work on to get even extension. As Sue says, once your knee has been operated on it is no longer the same. There will be some scarring within the soft tissues of the knee and no matter how good the surgeon the placement of the graft will not be exactly the same. Also bear in mind that your surgeon has to anticipate how your own tissues will respond over time. They tighten the graft in OR so can check tension on the day, but they have to guess how your body responds and what the tension is likely to be like in 6 months or longer. Sometimes grafts can loosen up.
Just keep plugging away at the rehab!