From Plan to defeat Dagoth Ur:
For the past twenty years the Tribunal have tried unsuccessfully to execute this plan. However, we failed because we were required to stage an assault and simultaneously maintain the Ghostfence to prevent the threatened large-scale breakout of Dagoth Ur's blighted hosts. With the Nerevarine leading the assault, and the Tribunal free to devote their full energies to maintaining the Ghostfence, this plan has a greater chance of success. Unfortunately, however, the loss of the artifacts Sunder and Keening, and the recent increase in Dagoth Ur's strength, poses new problems for the execution of the plan.
Because of Tribunal's ability to be done before the Morrowind main quest we have to assume Sil was dead and Almalexia has gone to bananas to lend her faiding power to it. So yes it's likely it was just Vivec at that point.
I don't really see it that way as the expansions were meant to be played
after the vanilla MQ. Doing them out of order makes it confusing when it comes to continuity. For instance, go and do Tribunal before you do anything else and you will still be referred to as the Nerevarine (by some, including Almalexia), without having to do any vanilla quests. Do the quests in order, as they were meant to be in order to keep continuity.
Therefore I assume, rightly so, that
at least Vivec and Almalexia are sustaining the gate. Who knows how long Sotha Sil has been dead? It could have been a decade, or more. So I think it is safe to assume he has nothing to do with maintaining the Ghostfence at the time of the MW MQ.