Silumgar's control-changing effect has a duration of "for as long as you control Dragonlord Silumgar". So stealing Silumgar with Control Magic means this effect ends.
Confiscation Coup's effect has no duration, so it is still "on" even while there was another effect taking precedence over it. Now that this other effect is gone, Coup's effect is the only one to applied, so you now control Merfolk Looter again.
Gaining control of Sower does not give you back control of Dromoka. In your example nothing causes the opponent to lose control of Dromoka so they control it the entire time. If you change your situation to Silumgar gaining control of Dromoka then control will revert to the opponent when the Silumgar's effect ends when the second sower gains control of Silumgar.
Confiscation Coup's effect has no duration, so it is still "on" even while there was another effect taking precedence over it. Now that this other effect is gone, Coup's effect is the only one to applied, so you now control Merfolk Looter again.