Thanks again for the input, all. I did the conversion today. Flip-flopped back to the Roth in-plan conversion. The big factors (aside from the ones outlined in my original post) were:
1) I determined that I can keep my plan after leaving my employer without any increase in fees. That can change, but I don't expect it to.
2) Confirmed that after leaving my employer I can make Roth principle withdrawals directly from the Roth 401(k). Conversions before then will season within the 401(k) as normal. There's no need to do a second rollover to an IRA and reset seasoning timeframes, so long as I keep my money in the plan.
3) Liability protection. I don't live in a state that extends similar protections to IRAs.
The last is the lynchpin. I was already planning on trying to defer withdrawals from tax-advantaged accounts for as long as possible. As taxable assets dwindle, we'll get more and more immune to lawsuits. Eventually, that should save a chunk on insurance every year. Immediately, the damage potential from a huge judgment will be reduced somewhat.
I'm gambling a bit that the things I like about my plan won't change. The call is borderline enough that I'm not too worried about that. It's a wager, but downside is limited.