We always remind employers it is not the employer's place to discuss how to prepare a W4. That is what the employee's tax person can be paid to help with, if the employee needs help. Looking at the form, I suspect most "normal" employees will not need special instruction, provided they even submit a new form. The other employees have likely been dealing with management of their withholding before, and have the means to create a valid new form.
For the calculation, the IRS is giving a clue (via the W4 instructions) how the calculations will be different, with a section titles "Instructions for Employer". For computerized payroll, no real problem calculating. For those using tables, more steps, but speed is not likely important to those using tables :).
The tables will still have allowance columns, as it was important to not require new W4 forms.
Spit balling, I will probably add W4 setup for the new W4 "lines". The existing field to enter allowances can remain, as a visual clue as to the 2019 defaults, and to also show the value entered prior to 2019. If any numbers are added in the new lines, the allowances will be set to the defaults. I would rather not show the allowances if a new W4 is used, but many employees like to check their checks, and most will use the tables. I think for clarity, the new line figures will need to somehow be reflected on the stub, meaning the employee will have the means to reverse calculate their withholding. Not sure exactly how yet, but it should not be difficult. Probably some way to show the wages, the adjustments, and the total used for taxable.