Scott if I am reading your research correctly, a rough timeline would go:
1771-1776 in Lititz as apprentice to Albrecht
August 1776 to 1780 - at Christian's Spring, moved to Nazareth 1780
By @ 1781 - had house/shop built in Nazareth for gunstocking work
Took younger brother as apprentice in 1781
Between 1781 and 1792, at some point not determined, seems to go from gunstocking to primarily joiners work and has apprentices in joiners work by 1792
Takes on an apprentice in joiners work in 1797
1798 accepts contract from state for 2000 guns, builds factory at Jacobsburg which is complete and operational by spring of 1799.
According to your writing, however, it appears there is evidence to support the idea that the Jacobsburg factory was solely for completing the state contract, at which point it was then converted to a grist mill. Meanwhile, he maintained a shop in Nazareth and may have expanded on it in 1800. He had various apprentices and other workmen employed or training under him there on a smaller scale until 1808, at which point he accepted another huge contract for arms.
So where to place this rifle in time and space? Well assuming it is indeed a William Henry rifle (no reason to doubt this, just can't believe they didn't photograph the signature!) I would proffer that it was likely made in Nazareth sometime between the two large arms contracts. The furnishings used here are a very well developed Northampton/Lehigh style of a later generation form - they are not the earlier style seen on the earliest Moll rifles or the dated Rupp pieces. The triggerguard in particular I would have a hard time accepting as pre-dating @ 1800 largely due to the way it's designed and filed. Also, assuming that the artificial striping is original to the rifle, I definitely do not see it as becoming popularly used in this region until the early 19th century.
As a random thought, this rifle is very well designed and accomplished but is at the same time fairly plain and utilitarian. The cheek star and the fantastic box are really the standout work here, but the lack of any carving and the 'quick' application of the fake striping and simple red varnish imply to me almost something of a - dare I say? - factory product! I find this line of thought interesting also because we're talking about a guy who clearly operated at various points with something of a factory mindset in order to complete the contracts. I wonder if what we have here is a rifle largely stocked and completed by apprentices or employed workmen and marked with the name of the "brand" as something of a commercial product, in other words not the more romanticized idea of a lone craftsman turning out unique pieces?
