Most of these people don't seem to get that Guan Yu almost certainly used a spear like most normal people at that time. There is no archaeological or textual evidence of anyone using a "guan dao" in the Three Kingdoms and it's unlikely that Guan Yu had his hands on a top-secret advanced prototype weapon. As with most historical figures in that era, many aspects of Guan Yu's life became highly fictionalised over the years, in part because he was a larger-than-life type of person even when he was alive.
The Qing govermnent may indeed have propped up Guan Yu for the reasons you described but they also tried to encourage national unity by likening Chinese ethnic groups to the three sworn brothers. Guan-worship predates the Qing dynasty by a long time, the Qing were just trying to take advantage of a pre-established trend.