There is no definitive document supporting one way or the other. It is more a matter of interpretation of the sources we have and personnal opinion
Because attuning himself to the Heart is not instant, and dispatching Nerevar would be too likely to trigger an interruption before the process could be completed.
As for the supposed loyalty of Voryn Dagoth, he brough a host of pretty steamed up Nords on the battlefield without informing Nerevar, who supposed them to be allied with the dwemers. Turning a two-way battle into a messy threesome without informing your leader does not appear very "loyal" to me...
Also he attuned himself pretty quickly to the Heart where Sotha Sil (who event at the time was not exactly an amateur in matters of magic) would spend years before feeling ready to attempt the experiment himself. This shows a pretty detailed knowledge of the workings of the Tools and the Heart that incite me to view Voryn as having planned a large part of the whole mess from the start.
Could be self-delusion - betrayed friend is a more appealing role than traitor. Could be an outright lie to attempt to sway the Nerevarine and lure him into giving away the Tools. Could be "guilt by association" as the Tribunal ended up doing what he did, but Nerevar turned against him while he still trusted them ?
I submit that all of this is interpretation and personal opinion.
Your doubt over the 'supposed' loyalty of Voryn Dagoth relies on the fact that
The Five Songs of King Wulfharth is a reliable source. This just isn't the case:
Wulfharth says that
"Nerevar carried Keening, a dagger made of the sounds of the shadows of the moon. His champions were Dumac Dwarfking, who carried a hammer of divine mass, and Alandro Sul, who was the immortal son of Azura and wore the Wraith Mail."This clearly states that Nerevar and Dumac Dwarfking were allies when every other source states that they were friends turned bitter enemies by the battle and that Nerevar slew Dwarfking.
Wulfharth says that these three (Nerevar, Dwarfking, and Sul) fought Lorkhan, Wulfharth and Dagoth Ur. That Wulfharth was mortally wounded by Sul but not before 'shouting Sul blind'. It states that Dagoth Ur (not Nerevar) slew Dwarfking, but not before Sunder 'struck his lord's heart' (I assume this to mean Lorkhan). This caused Nerevar to turn away from Lorkhan and 'strike down' Dagoth Ur (which we know to be false), 'but he took a mortal wound from Lorkhan in return.'
Nowhere, I repeat nowhere, does
Wulfharth even mention Kagrenac, or the disappearance of the Dwarves. To me, this makes it a dubious source at best.
I admit that
The Real Nerevar states that House Dagoth was allied with House Dwemer, but if this were true then it doesn't make sense for Nerevar to leave the tools with Voryn Dagoth after the battle.
As for Voryn Dagoth's mastery of the tools, I was under the impression that he was
corrupted by the tools, not necessarily that he had discovered how to use them. If he had truly figured out in a matter of, say, days what it took Sotha Sil years to discover it seems unlikely that Nerevar and the Tribunal would have been able to separate him from the tools. It also seems unlikely that he would have remained dormant under Red Mountain for thousands of years before re-emerging.