I received my A10 two days ago and as I'm totally new to 3D printing, there is already things I don't get

Here are the few points I'd like some help with, please.
- I realized that the bed isn't really centered. If I level the border on the left-front and pull the bed to level the left-bottom corner, it's not on the corner, it's a bit more on the right inside the bed... Is there a way to correct that?
- When I level (for example) the left-front borden and moves the extruder to the right, it seems to scratch the bed a bit, but the right-front border isn't too high, it's like the bed is not totally flat...
- When I home the extruder, it doesn't go on the left-front corner, it goes outside of the bed, is it normal?
- I ran out of the test filament and wanted to push filament from a new spool while it was printing, but once the test filament passed the gear that pushes the filament into the extruder, it stopped moving forward, even with the new filament pushing on it at the back... Now it's stuck... I don't want to break anything, so what's the best way to remove it. And for my information, what is the good practice when it comes to change filament during a print?
Thanks!

Tom