Regardless of when the ability is activated, it will apply to the defending player at the time Soltari Guerrillas tries to deal combat damage to them.

Multiple instances of shadow on the same creature are redundant.

Once a creature has been blocked, that creature remains blocked and will deal and be dealt combat damage even if it gains or loses shadow or if the blocking creature gains or loses shadow.

If an attacking creature has multiple evasion abilities, such as shadow and flying, a creature can block it only if that creature satisfies all of the appropriate evasion abilities.