I was under the impression sweeping would move the coins off-chain without incurring any possible transaction fee, and would do this instantly without need for confirmations. The more I think about it, the dumber that sounds as the rest of the network would have no idea those coins were "moved" in the first place.

I'm now a bit confused as to why someone would wish to sweep coins at all. It seems you need to have total control over both wallets in either scenario. You're also advising to never "delete" a wallet. If sweeping still means the coins get sent as a transaction to a new address, how is this different than a normal send?

In my question I do not care at all about signing messages or proving ownership at a later date. I'm more interested in keeping funds secure and organized. I also do not care about the astronomically remote chance of a private key collision.

So, the million dollar question: Why sweep at all, ever? Would the best course of action to be keeping all of the older wallets empty after draining them with a single transaction to the armory wallet, which would consolidate everything?