How to handle form validations when the Cancel button can lead into an incorrect state?

Question

I'm designing a set of screens where the user inputs certain data on Screen #1, Screen #2, Screen #3, etc. At the end of their journey they're presented with a final review screen, where the user can confirm that their input is correct. If they want to correct a value, they can click on a "edit" button next to the field, similar to how many review screens are designed:

Here's the crux of my problem. Let's say that I have a validation rule requiring that the Delivery Address and the Billing Address must be within the same country. Now the user can have the following flow:

1. Open both Delivery Address and Billing Address sections
2. Change the country in both
3. Click Save on Delivery Address
4. Click Cancel on Billing Address
5. Since Cancel reverts the form section to the previous state, I am now in an invalid state where the countries mismatch between the two sections

How can I avoid this scenario? Some options I've thought of:

1. Require that sections are edited one at a time -> inconvenience for the user (?)
2. Enforce validations on Cancel button -> weird UX (?)
3. Prevent saving a state that might result in an error state -> weird UX and easy to get wrong in the JS code (?)

Answer 1

Best I can think of is:

a) While inputing fields in the first place, after user has filled in Delivery address and came to Billing address; Billing adrress country field should be preselected, easily greyed out, and small warning should be shown below the field with "Billing address country should be same as Delivery country"

b) You can preserve same logic on editing. Billing country is always preselected, and user can change it on the part with Delivery. On editing screens then again small notice under Delivery country field with "Billing country will be auto updated" is needed. Please update your billing info"

This all goes by the logic that "Delivery address is more important for your users than payment method". Or that users will rather and easier adjust payment method address to delivery address, than other direction.