Work around found:
Fresh/clean install of Arduino Install M5Core2 in Library Manager & opt to include all 15 dependencies (will fail with "no protocol:" error) Install manually the 5 remaining dependencies that failed (MFRC522_I2C,M5_BM8563,M5_ADS1100,M5_ADS1115,HX711 Arduino Library,PCA9554) Install M5Core2 v0.0.1 in Library Manager (downgrade) Update M5Core2 in Library Manager to latest version (0.0.6), will list 0 dependencies and ask if you would like to install them, but select "Install 'M5Core2' only".Can now compile and run helloworld example (and others too). Confirmed same install problems encountered on a second PC, and same work around solved it.
Arrgh~! Shouldn't be this messy/difficult.