I agree, but have we not established that these dwarves probably did not come on the direct orders of Dáin? They themselves say that they are from the Iron Hills, which is not under Dáin's command from Erebor (see the Hobbit- Dáin rules there, the dwarves over there belong to him, and any messages that need to be exchanged had to be by raven). I believe that these dwarves probably went of their own whim. There aren't nearly enough to actually reclaim Moria. They also aren't enough to have been missed in battle when they were gone. So I guess that a dwarven expedition was formed months ago in the Iron Hills, before the Nazgûl came to Dáin. They probably also left a little time before Glóin did from Erebor. That means that they would have left before word arrived of Dáin's plight, but being large in numbers, would probably have been behind Glóin's party and arrived at Rivendell after the Fellowship had gone south. Likely they used the same High Pass that Thorin and Co. used in the Hobbit, thus arriving at Rivendell and then making the journey south from there through most of Eregion to Hollin Gate.