Sorry mate, I really made a mess of my response! After reading it again I deleted my reply. It was totally off.
I've had a look at the FastTrack Pro.
According to the manual, the mono/stereo button on the front panel is used for input monitoring. I don't "think" it will affect whether the source is recorded in mono or stereo. So don't think this will fix the problem.
You have two input channels on the fasttrack pro. One would be LEFT and the other RIGHT. Did you check if, that when you select a new audio track in Mixcraft to record to, that it is not a STEREO audio track? I still feel this is the problem. It should be a MONO audio track.
If this is the case then by plugging the guitar into the OTHER input and recording, the sound will only play from the OTHER speaker this time.
Try it and see if this is the case.
Oh by the way you mentioned getting a weak microphone signal when you tried to record vocals? What's the connector on your mic cable? 1/4" (TS) or XLR? The manual states you can use either but I don't know about that, since the TRS inputs are meant for Instrument and Line levels. Use an XLR cable if you're not already. I have a hunch that will fix it. Oh also, what are the monitors you are using? Make sure to use the correct outputs on the back of the FastTrack Pro. The TRS outputs are balanced, whereas the white and red RCA jacks labelled (1 & 2) are the unbalanced outputs.
It's 5:54am here in Sri Lanka. I gotta go crash. Sorry again for the misinterpretation of your original post. Hope I haven't talked trash yet again.
I shouldn't be replying in this state.
Sriyan.