How to automate sync of 80-120 audio clips recorded at home on mobile phones?
At the start of the accompaniment track, about 15 seconds before the music starts, record a clap, or other loud but brief sound. Have your choir members record this clap sound from the accompaniment track onto their contributed recording, but not record the rest of the accompaniment. 

You can do this by having the singers put on their headphones, but unplug the jack from the playback computer.  They start recording on their phone, then start playing back the accompaniment track. The sound of the clap will be played out loud, and captured on their recording, because the headphones are unplugged. Immediate after the clap, the singer plugs in their headphones. This stops any more of the accompaniment from being audible on the recording. (Important detail: experiment with how much time you need to leave between the clap and the start of the accompaniment, so that singers can plug in their headphones and get ready to sing.)

When the singers finish the piece, they stop recording, and send you their recordings. You need some system for naming the audio files, and tracking the identity and vocal part of each singer. (The details of this tracking are not germane to the sync question.)

Make a copy of the first few seconds of each contributed recording in .WAV format. Write a Python program to detect when the clap occurs relative to the start of a recording. Do this by reading the value of each audio sample of the .WAV file, and identifying the sample with the largest value. This is the moment of loudest sound. Convert the position of this largest value into an offset of seconds from the start of that recording. 

Now, merge the recordings, applying an offset to each so that the clap occurs at the same moment for all all recordings. The recordings should now be in sync. Mix the recordings to taste, and save the mixed recording. 

[Note: this is a speculative answer, based on reading Virtual Choir by Jed Scott, and CB Multitrack Editing 101 by Christopher Bill. Most of the virtual choir instructions I found on the web did not ask the contributors to record any sync moment at all. I guess they just have a human align all the parts by ear.]