1) Turn my Roth IRA into a Spousal IRA and double my contributions (for two people instead of one).
Why not just open a separate IRA for the Spouse? I've never seen a single IRA combined for two people.
2) Increase my Roth 401k contributions and discontinue my traditional 401k contributions for the time being (we'll be squarely in the 15% bracket, and I don't think it'll get any better than that).
That's a pretty good idea. Like you said, you'll likely never be in a lower bracket, so take advantage of that.
This is the wild idea: 3) Roll my traditional 401k into my Roth 401k and pay the taxes on it.
For idea 3, before getting married, I used my traditional 401k to avoid entry into the 25% tax bracket. Since I'll have plenty of wiggle room after getting married, I'll be able to convert my traditional 401k contributions to Roth contributions, and pay just 15% tax to do so. Is this correct? Someone please tell me I'm making sense there.
I like this idea too. Note, that the conversion will increase your income that year for the amount converted, so it may push you into a higher bracket. Also, be sure you have enough money OUTSIDE of the 401(k) to pay the tax on the conversion. If you use money from within the 401(k) to pay the tax, that is considered a distribution and will be subject to early withdraw penalties.*
*That's at least how it works for TIRA to Roth IRA conversions, I've never looked specifically at 401(k) to Roth 401(k) conversions, but I assume the rules are the same.