I mean that there are some issues here, and I'd have to type out an itemized list to address them all.
Let's start with Sinclair. He was born two generations after the Templar order had been dissolved. He was not a Templar (and could not have been, as they no longer existed), but I suppose that he could have been an ally of the children and grandchildren of ex-Templars that fled to the Orkneys. But why?
He was a jarl (or earl, if you prefer the anglicized version) and held this title at the pleasure of the king of Norway. This was an important title, and it could be taken away. A jarl/earl was required to be ready to quickly (for the time, anyway) respond to the king in person, with retainers, in order to support him. Jarls admittedly did not always meet expectations; when this happened, the king outlawed them. Outlawry in the lands under Norse influence at that time (to include the Orkneys) was a big deal, as it involved being stripped of all of your land and wealth and required your departure. If you refused to comply, it was not only legal for a free man to kill you, but it was actually his obligation to do so. Taking down a former jarl in a remote part of the kingdom would not be easy, but it happened occasionally in the sagas (Gunnar's rise to glory in Njal's saga comes to mind) and would guarantee not only wealth and honor, but more importantly the friendship and gratitude of the king. Sailing into remote places in order to kill people and take their things was what these people did for fun and profit during that time, and some of them were rather good at it. I would not want to be an outlawed jarl who refused to leave. Somewhat ironically, it was the refusal to leave Iceland after being outlawed that resulted in the death of Gunnar, who I mentioned previously.
Sinclair was not an outlaw because if he had been, it would have been recorded. Thus, we can assume that the king had no reason to dislike him, particularly when some of his family members were bucking for the same title. Therefore, if he disappeared for a year or two, it was with the king's blessing. Such a disappearance is not documented. It's possible that such a record didn't survive until modern times, but it's equally possible that such a record was never made. But why not? There was no reason to hide it. Norsemen and Icelanders had been making journeys to the New World for centuries by that time. It was no secret. Likewise, he'd need a reason to go. If he was hauling treasure for Templars (or Freemasons, or aliens, or whatever), did he tell the king that, or did he come up with another reason? But there's no good reason for a jarl to go on such a long and dangerous voyage himself unless he's personally supervising something of vast importance, so he either told the king or he sent someone else. If he told the king, we must now explain what the king's interest in this venture was. If he sent someone else, Zichmni goes out the window.
So what's going on here? He probably didn't go on his own, but the king doesn't appear to have sent him. There are lots of problems with this, and it's only one part.