If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.

In the incredibly rare case where there is no legal target for the first or second mode, those modes can't be chosen at random. Since the third mode doesn't require a target, it is always eligible to be chosen.

If a permanent that would be dealt damage has more than one shield counter on it, that damage is prevented and only one shield counter is removed.

As you put one of Summon: Magus Sisters's chapter abilities on the stack, you choose a mode at random. Players can respond to the ability knowing which mode was chosen.

A creature with a shield counter on it may still be destroyed by state-based actions if it has damage marked on it equal to its toughness or has been dealt unpreventable damage by a source with deathtouch.

Removing a shield counter when a permanent would be dealt damage or destroyed isn't the same as regenerating that permanent.

Shield counters don't prevent players from sacrificing creatures.

The random choice of mode for Summon: Magus Sisters's chapter abilities is made before targets are chosen.

"Shield" is not an ability that creatures have and shield counters are not keyword counters. If a creature with a shield counter loses its abilities, the shield counter will still protect it as normal.