Ferguson disputed all four pieces of evidence cited by Rupke and concluded that Rupke's case for allochthony is weak. This paper also refers to the work done by Dawson and describes Sigillaria as being preserved in situ. This is still the prevailing view of the fossils found here and this interpretation is not as hotly debated as you would like people to believe. Just because polystrate fossils in one area are allochthonous, it doesn't mean that all polystrate fossils are allochthonous. That's faulty reasoning.
Every site has to be examined closely.
One must consider all of the paleontological and lithological evidence when interpreting an area. You mentioned Spirit Lake and Mt. St. Helens earlier as a locality where polystrate fossils might be forming. Maybe not a single global flood, but possibly not in situ either. Sure, these Sigillaria might not be in situ. It's the same argument I use when arguing with Art Chadwick.