Arkansas does not allow local jurisdictions like cities or counties to collect a local sales tax. The Arkansas state sales tax of 6.5% is applicable statewide. You can view the sales tax rates for various cities in Arkansas here.
Equitable adjustment is a term of art that includes an adjustment to fee, and the target. minimum, and maximum fees should not be increased in this case for what I hope are obvious reasons. The adjustment should be limited to the effect of the new wage determination on wages and fringes.
I applaud your thinking that there is a difference between a request for equitable adjustment, on the one hand, and a definitization proposal submitted at the government's direction, on the other, and that DFARS 243.204-70 and the clause at DFARS 252.243-7002 do not apply to a definitization proposal.