1) You would report it on lines 4c and 4d. A 401k is a qualified plan treated as a pension. Lines 4a and 4b are for IRAs only. Your 1040 would show the total distribution on 4c and 4d would be blank. “Rollover” should be entered just to the right of “taxable amount.
2) The total of 13,006 goes on line 4a because you meet the exception of converting to a Roth (Exception 2). The taxable amount of the conversions goes on 4b, but is calculated on Form 8606 before entering the taxable amount on 4b. Each spouse has their own 8606.
3) No explanation needed. The 8606 forms take care of that. You do not have to enter “conversion” or “rollover” next to 4b.
4) 4b should show only $6 taxable. However, if you are using a tax program you might get into some rounding differences for the non taxable decimal on Form 8606. The non taxable % should be carried out to 4 decimal places to get the most accurate result.
5) This response assumes that neither of you have a non Roth IRA balance on 12/31/2019 and have to other 1099R forms for TIRA distributions.
6) Line 14 of each 8606 should be blank with no IRA basis carried over to 2020.