AmNike

There are two problems I have with your tl;dr version of this. First, we know that Tanavast merged is Cognitive Shadow with Stormfather as he began to die, meaning that Stormfather isn't actually acting like Stormfather (the being that has existed since before the Shattering). Instead, Stormfather is acting like Tanavast. With Fuzz, we have a way to argue that this fake Stormfather could simply be another splinter of Honor that retained parts of the personality of Tanavast. We know that by the time of his supposed death, Tanavast was raving mad, more concerned with obeying the spirit of Honor than the law of Honor. This allows him to lie and act ill tempered as we see him do in the prologue. Second, we have Fuzz to show that we can have a Shard manifest in a similar way to their primary manifestation of Investiture. Fuzz turns into a mist wraith on the physical realm, so it would make a lot of sense for Tanavast's splinter to manifest as a spren-like human figure made of light, not quite distinct enough to have a full form.

This is what I was initially thinking as well. The inability to manifest fully in a human-like form seems pretty similar to Fuzz, but possibly manifesting in the Roshar version of it. I mean, if Preservation/Fuzz manifests in the mists, then it would make a lot of sense for Honor/Tanavast to manifest in a shimmer.

This actually makes a lot of sense because of how splintered shards can't really appear all that well in the physical realm. While Preservation appeared as a mist-wraith-esque figure as he fades away, Honor could realistically appear as the not-quite-visable silhouette that Brandon describes.

We know that Honor went a little crazy before Tanavast actually kicked the bucket, so it's possible that the Stormfather-esque figure could be a byproduct of the splintering of Honor. We know that splinters can take on a mind of their own, and while Tanavast merged his Cognitive Shadow with the Stormfather, that doesn't mean that part of the Investiture didn't also take on sentience post-splintering. In other words, we could be dealing with a small portion of Honor's Investiture that mirrors his madness towards the end of his life. This would explain why he can also show the visions, but also why he lies: he's following the spirit of Honor's beliefs while breaking the law of the beliefs by lying.