1st check to see if the midi file is format 0 or format 1.
Format 0 puts all of the tracks into 1 track and that makes a mess when trying to edit the file or import it into musescore.
Format 1 separates the tracks. If you don't know which format the file is, use Anvil Studio (it is freeware) to load the midi file. If it is a format 0, anvil studio will convert it to format 1 before it loads it. Once it is loaded, you can save as... a different name and it will be saved in format 1.
I use anvil studio just for this reason. Once it is loaded, it is also very easy to adjust the panning and volume of the individual tracks, then I save it and use Aria Maestosa (also freeware) to do the final editing.
I have used many midi editors, but find aria maestosa the quickest and easiest to use.