Yes, 1, 2 and 3 are all correct. The $5,500 that results on line 14 of the 2015 Form 8606 carries forward to line 2 of the 2020 Form 8606 to be added to that year's nondeductible contribution and the $11,500 result on that form's line 14 carries forward to line 2 of your 2021 Form 8606 to be combined with the $6,000 nondeductible contribution for 2021 and used in calculating the taxable amount of your Roth conversion.
Since you apparently still had some funds in a traditional IRA at the end of 2021, only some of your basis in nondeductible traditional IRA contributions will be applied to the Roth conversion with the remainder of the basis to be applied to subsequent distributions from your traditional IRAs. If the amount rolled over from the 401(k) to your traditional IRAs was greater than $17,500 and all pre-tax, more than half of your 2021 Roth conversion will be taxable.
