Don't feel sheepish about sliding down a slippery slope because that's it exactly, slippery. Everyone will slide back down on a slippery slope. Mr. Henry and I used MIDI files of commercial recordings because they are familiar to a wide audience and are easily recognizable as authentic, accurate and correct sounding. The MIDI content contained in the files regardless where or how it originates is not relevant. BIAB will process the MIDI data the same.
I have also used many custom MIDI files created by either of two local professional keyboard players that range from complete productions of a commercial release recording for which there's not a commercial available MIDI song file to be purchased, to individual backing instruments playing specific parts of a song and even only several measures of a specific riff. Your MIDI file composed of a single instrument will be identified, imported and processed the same as a commercially produced retail product in regards to how it will operate within the BIAB program.
Using readily available complete MIDI files simply makes the process easier and allows you to concentrate most of your focus on the song and applying BIAB techniques and processes. The familiarity and groove of the song keeps you from worrying about the sound. The next step, (another video by Henry Clark) expands the process from copying a song to covering a song by using styles and arrangement so the song no longer sounds exact and correct but is modified by you to be your own personal interpretation of a song you're familiar with. The final step of this method prepares you to create your own custom song from scratch using the tools you learned and employed to create the first two steps.
The barriers you face and the processes you don't grasp are the same issues others will have. Overcoming your obstacles will help others overcome theirs. I suggest you use Mr. Henry's video as a common denominator to a question/answer discussion in this thread where you can direct your question relative to the spot in the video you're having problems getting to work or understand and have the issue discussed here on the forum and once that point is resolved, move on to the next point in the video that you're struggling with. In the end, you and others that follow along will have compiled a complete song from beginning to end learning BIAB arranging and composing techniques along the way.