Coram's first ability applies only while it is on the battlefield. In all other zones, its power is 0.

The value of X will change as the greatest power among creature cards in all graveyards changes.

You pay all costs and follow all timing rules for lands played and spells cast this way.

Once you begin to cast a spell this way, losing control of Coram won't affect it. You can finish casting it as normal.

If you play a land or cast a spell from a graveyard using another permission, it won't stop you from using Coram's permission to do the same thing later in the turn, provided you haven't already done so.

If you play a land, cast a spell, or both using the permission granted by Coram's last ability and then have a new Coram come under your control in the same turn, you can still play a land, cast a spell, or both using the new Coram's permission.

If it's your turn, you control Coram, and another player has permission to play a card in a graveyard that was put there from a library this turn (perhaps because it's an instant card they own that has flashback), whoever gets priority sooner will have the first chance to play that card. In most cases, this will be you, since the active player receives priority after a spell or ability resolves.