I'm clear on the point that by transferring the pre-tax segment of the IRA into the 401(k), the remaining all-post-tax IRA could be Roth-converted without paying tax.
What I'm not clear on is that you couldn't do the same effective transaction without the 401(k) existing. (Which seems to be your point above.) Is it an issue with un-commingling the funds within the IRA?
Sorry for being slow here!
Steve