I think most of these issues are easy to resolve by being more permissive and supporting what the technical standard allows for.

The Big Problem™ however is case sensitivity in the local-part, because there multiple incompatible things collide:

1. Users are not universally aware of case (in)sensitivity in one direction or the other

2. Existing systems may or may not interpret case at all

My preferred solution would be to adjust the standard to ignore case in the local part by forcing it to lowercase. That aligns with most of the systems and mental model of technically proficient users anyways. It makes much more sense from an UX standpoint since the goal is to be imambiguous.

If we were to enforce the opposite: case sensitivity in the local part this would have multiple downsides:

1. It is inconsistent with itself by making the local part case sensitive but the host part not, that is harder to explain

2. You have to train users to be precise about case on entry. As someone who worked in IT-support, this is a very bad idea. This includes second-order issues like phishing attacks by silbling emails where just the case differs

3. If your service stores email addresses it will need to know whether that specific Mailserver/client/etc treats the email as case-sensitive or not

In my eyes email servers that allow case sensitive local-parts are functionally broken, even if they don't break any rules.