Hemalurgy can steal both Allomantic and Feruchemical powers, yes, but you can only steal one power from a person, just like you could only take one power from a Mistborn, because Hemalurgy (almost) always kills you in the process.
Does anyone have a quote saying Archivists no longer exist? Because I must have missed that.
There were Mistborn after a thousand years, yes, but there was no Feruchemy interfering with their sDNA, so that doesn't apply all that much. He did. He put everything that was in his copperminds into the Words of Founding.EDIT:
He reused the Feruchemical spikes. If they found a Feruchemist, they would of course spike him, but by this point they were mostly reused. Here's the WoB:
EDIT 2: edited for brevity.