Here, from pub 590A,  seems to be the pertinent point that helps my situation....

Applying excess contributions.

If contributions to your Roth IRA for a year were more than the limit, you can apply the excess contribution in 1 year to a later year if the contributions for that later year are less than the maximum allowed for that year.

Client has to pay the 6% for several back years, but, under the 'apply the excess contribution' scenario the problem will self correct in 2020....

thanks for your input