Re locked door: Just say it's for your protection coz when I was in KL, we would look our room doors (my parents, myself and my bro). At that time, there were a lot of break-ins and if the doors are locked, at least you buy yourself some time coz you can call the police and so forth. Or next time she bath ar, you stand in front of the bathroom and yell for her. Then when she ask you why you do that, you say aiya, we are family, no need to close bathroom door . :P
Re stamp and time your mother in law can stay here: If she comes in my plane, one month. If by train or across JB, it's two weeks.
Re your hubby coming home late: Avoiding you and his mum isn't going to make the problem disappear. In fact, it makes it worse. Instead of scolding, he should sit down with her and ask his mum what she would like the CL to do, what she would like to do and so forth. Then share with his mum that with a baby, things are different now. Cannot always accommodate her all the time coz taking care of a baby is more important. You need your space and rest. Use this line "As a mum, I'm sure you can understand the situation".
Sometimes you need a clash or a fight for things to sink in. I had some issues too when my parents came over and things got really tough between my hubby and them. I was due then and it got to the point where I was crying coz my dad would say things like "If I know like this or that, I won't come here ". After that episode (staying with us for nearly two months), every time he and my mum comes here, they'll stay in a hotel. My mum, on the other hand, can stay with us and there won't be any issues. But my dad...hai.