Lithium-ion charge cutoff
Applying 4.2V indefinitely to a LiIon cell will damage or destroy it.
Worst case metallic Lithium may "plate out" and the classic "vent with flame" failure mode may occur.   

A LiIon cell is mechanically stressed by charge/discharge cycles due to the transport of Lithium ions in and out of the cell structure. [LiFePO4 cells almost eliminate this affect by providing a mechanical olivine "cage" into which the Li ions are "intercalated". 

A look at the charge/voltage curve for a charging LiIon cell shows that towards the end of charge the voltage rises at a rate which is increasingly greater than the rate of charge absorption. This reflects an increasingly inefficient charging process as the last available areas are used to 'store' ionic Lithium.

Bythe time Ich under CV mode has dropped to 10% of Imax the cell is almost completely charged to capacity - "road warrior" mode where maximum possible energy is stored at the expense of lifetime. Cells charge terminates at 0.1 x Icv will have a short cycle life. Terminating at 0.5 x Icv still achieves > 90% of maximum capacity and stopping at 0.25 x Icv is probably preferable to 0.1 x Icv in most cases.

Proper design of charging circuitry makes it easy to detect current into and out of the battery while load varies from 0 to 100%. The battery and charging circuitry are fed from the charger input point as is the load. The load current may affect the amount of current available for charging (depending on charger capacity) but battery current is monitored regardless of direction or magnitude.

When charging multiple cells in series a modern BMS monitors the voltages of ALL cells individually. When one or more cells in a string reach their final state of charge ahead of others balancing circuitry 'shunts' current around the cell so that the cell is effectively separated from the charging string.