Any question answered...

My front door is completely stuck because of the rain, as the bottom of the front door (I discovered about a month ago) is unsealed. I'm due to get it repainted/treated within the next few weeks, but is there anything I can do in the meantime?

Depends how stuck it is...

A simple sandpaper on a block, to the underside of the door may help, without taking the door off ...

If (only if) the door step can be sanded refinished, that is also another way to 'ease' the door. However that is not ideal.

In a perfect world door off, and powerplane a bit off the bottom. then paint-seal with 2-3 coats would be perfect as well as additional weatherstripping.

Also check the grading of ground outside if the level of the door is close to the ground.