1) Because the MP3 is a stereo file it splits into 2 tracks. One for left and one for right. If you delete one of them you will only hear either the left or the right channel. This means you’re going to miss the audio available on one of the channels. The only proper way to do this is to convert the audio to a mono track so all sounds from both left and right will be placed center/mono. Then you only have one proper single proper track to place in uTool.
2) I mostly use stereo playback to hear whether the tracks are playing back. You can use solo (S-button) or mute (M-button) to only hear certain tracks.