Lord Xander does not have summoning sickness.
An opponent has an even library size.

Declare Lord Xander as an attacker, attacking any opponent with an even library size.
Lord Xander's second ability triggers, causing the opponent to mill their entire library.

Fraying Sanity attached to an opponent with an even library size.
Lord Xander does not have summoning sickness.

Declare Lord Xander as an attacker, attacking the opponent enchanted by Fraying Sanity.
Lord Xander's second ability triggers, causing the opponent to mill cards equal to half their library size, rounded down.
At the beginning of your end step, Fraying Sanity triggers, causing the opponent enchanted by Fraying Sanity to mill cards equal to the number of cards put into their graveyard this turn.

Declare Lord Xander as an attacker, triggering its second ability, causing the defending player to mill half their library, rounded down.
Activate Keening Stone by paying and tapping it, causing the player to mill cards equal to the number of cards in their graveyard, thus milling the rest of their library.

If the opponent has an odd number of cards in their library and an empty graveyard, the opponent will have one card remaining in their library. Otherwise they will mill their entire library.

Declare Lord Xander as an attacker, triggering its second ability, causing the defending player to mill half their library, rounded up.
Activate Consuming Aberration by tapping it, causing the opponent to mill cards equal to the cards in opponents' graveyards, milling the remainder of their library.

Lord Xander does not have summoning sickness.
You have not activated Riverchurn Monument's exhaust ability.

Declare Lord Xander as an attacker, triggering its ability, causing an opponent to mill half their library, rounded down.
Activate Riverchurn Monument's exhaust ability by paying and tapping it, causing each player to mill cards equal to the number of cards in their graveyard.

Declare Lord Xander as an attacker attacking the opponent named by Sewer Nemesis. Lord Xander's second ability triggers, causing the opponent to mill half their library, rounded up.
Activate Sewer Nemesis by tapping it, causing the opponent to mill cards equal to the cards in their graveyard, milling the remainder of their library.