If I have understood your original post correctly you would like your bird to live in your room outside of the cage but your mum wont allow that, she will allow your bird to have out of cage time but not live full time out of the cage in your room- if this is the situation then I have to say I agree with your mum... a bedroom is not a safe place (for either of you really) for your bird to live uncaged as you likely have many many things in your room that your bird could be harmed or killed by... power cords and points, materials, many many hiding areas which can cause all kinds of issues, many people have aerosols, perfumes, make ups, batteries, nick knacks that may be unsafe, textas etc. most bedrooms are not a suitable place for a bird and birds being in your sleeping area is not ideal for your breathing health.
Either way- you mum likely pays for all bills including vet bills so I really think this is a matter you are best to discuss with her without pushing that she is wrong or unfair- she will have to weigh up what is best (as far as she understands) for you all and also what is a risk or unhealthy situation.
No bird left to roam free in a human environment is really very safe unless the human lives in a very odd way...