What about the pact Sotha Sil made? Isn't that still in effect?

Didn't Mehrunes Dagon say to Sotha Sil that the pact was pointless?

Where is that?

I suppose the pact could have been just for the duration of the war with the Empire...

I think the general idea of the "Planemeld" is that it's something new (even though the Oblivion Crisis is very similar). The newness being: Molag Bal doesn't invade the boundaries of the Mundus by force, but rather, through changes in his own sphere/realm/self/aura, and subtler changes to the path of Tamriel itself, they begin to resemble one another, this causing them to align on some mythic axis (hey, if the Ayleids could rule the world with a model of the cosmos...) and, like two droplets of water coming into contact, they then merge, depositing Molag Bal's self as the speck of detritus at the center of one of these droplets. Through the Anchors, and the perversion of the flow of souls in the Mundus, the bond between Coldharbor and Tamriel is strengthened.

If you buy any of that.

However Sotha Sil's pact failed to prevent the destruction of Mournhold. A Breton Witch summoned and dealt with Mehrunes Dagon for the destruction of Mournhold and its Duke. Sotha Sil and Almalexia ware too late to prevent the destruction, although they managed to send back the Prince of Destruction back to Oblivion.

Dagon broke the pact.

Did he?

From 2920:

It doesn't sound like it was ever really a binding contract- more of a gentleman's agreement, really.

A nameless Breton witch sounds like an amateur to me, though we can only guess.

If a deadra was to break the rules and nobody was there to enforce it would the rule still matter? Who's the say the tribunal aren't already preoccupied with other matters. Also we have to consider that the Tharns and Mannimarco are behind the invasion.