The Creuss Flagship Hil Colish also has this rule:
"This ship may not use the D wormhole it generates." So the Flagship obviously acts only as a Wormhole and cannot go into itself, making it an illegal move.
Fighters can fight without the capacity needed to support them for the duration of the battle. For example, if you have a Carrier with 6 fighters and the Carrier is destroyed, the Fighters go on following the normal Space Battle rules. In the end of the whole Space Battle (not just the battle round, the whole battle) the unsupported Fighters are discarded. So I guess your Ground Force-Fighters just return to the planet they were on after the battle.
About playing multiple Action Cards: A player may never play two identical Action Cards for the same situation and/or on the same entity during one round. Example: A player cannot play two "Flank Speed" Action Cards on the same fleet in one round. The player may, however, play a “Flank Speed” on two different fleets in the same round.
So the answer to Signal Jamming would be that you can play it on different Systems, regardless of who is the owner of the System. Now if the Signal Jamming card targeted a player instead of a System, you could not play it twice on the same player.
EDIT: About the Shard of the Throne political card:
From the wording of the card, I understand that the card indeed goes back and forth between the two players. Player B, owner of the Shard of the Throne, gets one of his ships destroyed by Player A's ship. Player A gets the card. I think it doesn't refer to the player who attacked the System, but the ship attacking and destroying another ship.
Hope this answered your questions. Hopefully it's all correct, too