Does the Device need to be in assembled, (ready to use condition) when it is determined if it, along with any tools and/or supplies, fits in the 100 x 100 x 50.0 cm box? Or is it permitted to be partially disassembled (such as removing the levers)...

It only needs to fit into the box at impound, however note that there isn't any explicit time stated in the rules to allow for assembly, thus it will be up to individual event supervisors as to whether time is given to all teams, or if a team must assemble during their part 2 testing time.