Ummmm…based on the contribution limit you posted, it’s just Head of Household. Single is a different filing status.
(a) contribute $2,000 to a traditional IRA, i.e., the remaining balance of the $6,000 limit for 2021 less the $4,000 Roth IRA contribution)?
Yes. Presuming the individual has at least $6,000 in compensation, total allowed contributions for this individual are $6,000, which can be split between Traditional and Roth IRAs as desired.
(b enter a $2,000 IRA deduction (for the $2,000 traditional IRA contribution) on Schedule 1 (Form 1040) 2021, Part II Adjustments to Income, line 20?
No. From Table 1-2 in IRS Pub 590-A https://www.irs.gov/pub/irs-pdf/p590a.pdf if your filing status is either Single or Head of Household, you cannot deduct any Traditional IRA contributions if your income is greater than $76,000 So the $2,000 Traditional IRA contribution that you are proposing above would be an after-tax contribution. If the individual has NO other pre-tax (including but not limited to deductible, rollover and earnings that no taxes have been paid on) balances in any Traditional IRAs (including, but not limited to, Rollover, SEP and SIMPLE), then they can do a conversion of the $2,000 into a Roth IRA with little or no tax consequences. If they have other pre-tax balances, the conversion will be taxed on a pro-rata basis. As an example, if the individual has a total of $20,000 in Traditional IRA balances including the $2,000 after-tax contribution, any conversion would be taxed on 90% [($20k - $2k)/$20k} of the conversion.