Yes, fed estimated payments are fixed for the date they are due. 

If your client wants to make them on a different day, you can use IRS DirectPay and schedule them for whatever dates you'd like rather than use the software.

Im not familiar with MD, but if youre not seeing the option on the MD ES worksheet, chances are PS doesnt support it.

Proseries does not support debits for Maryland individual estimates.  I have been asking them for at least 5 years and it falls on deaf ears.  There is really no excuse except they want to continue be have an inferior product.  You can setup your clients estimates on Maryland Ifile website.