In both cases I would say there is good chance of being held liable. In neither case would I expect a charge of "attempted murder." But that's what is being asserted in this thread, per the OP. The OP asserts that the persons who built the gallows art built it for the purpose of hanging someone. That's quite different than the scenarios you just offered.
It seems that in Plan A, its purpose was to
threaten Mike Pence into going along with Trump's "fake electors" coup attempt.
Plan B would have been to kill Mike Pence and have Trump choose a replacement (via the process in the 25th Amendment) who would be willing to carry out the VP's part of Trump's coup attempt, then proceed with the coup. However, I think that if the mob had got their hands on Pence, it's questionable whether they would have had the presence of mind to haul him out to the gallows rather than kill him on the spot. There certainly would have been...
logistical challenges in hauling Pence outside without being stopped.
... so really, I'd say the gallows' real purpose was to look effective enough to scare the crap out of Mike Pence. Being an actually usable gallows was secondary (edit: but still related to its main purpose).