Explanation
The best practice when multiple users are editing an email template at the same time is to always click Save and Exit or Save Draft and Exit to commit your changes and close your session before someone else edits it.
This way, you can avoid overwriting or losing your work, and ensure that the email template is consistent and accurate. If you do not save and exit, the system will warn you that another user is editing the same email template and ask you to confirm your changes

According to the Salesforce documentation, in order to sync a Marketing Cloud Account Engagement prospect field with both a lead and a contact field in Salesforce, the lead and contact fields must have the same API name, and the Marketing Cloud Account Engagement field must be mapped to the Salesforce field. The API name is the unique identifier for the field that is used by the system to sync the data. The field mapping is the configuration that tells Marketing Cloud Account Engagement which Salesforce field to sync with. The Salesforce field label and the Marketing Cloud Account Engagement field type are not relevant for the field sync, as long as they are compatible. Reference: Salesforce documentation

Explanation
By default, if the information in Salesforce differs from the information in Marketing Cloud Account Engagement, the information in Salesforce will override the information in Marketing Cloud Account Engagement. This means that Salesforce is the master source of truth for most fields, and any changes made in Salesforce will overwrite the values in Marketing Cloud Account Engagement. However, this behavior can be customized for each field in Marketing Cloud Account Engagement, by choosing one of the following options:
Use Marketing Cloud Account Engagement's value, Use Salesforce's value, or Use the most recently updated record. You can also prevent a field from syncing at all by choosing Do not sync. For more details -> 45

Explanation
A segmentation rule is a type of rule that allows marketers to segment their prospects based on specific criteria, such as field values, activities, or scores. A segmentation rule runs only once, as soon as the user completes the rule creation process. The rule will not run again, unless the user manually re-runs it or schedules it to run at a later date. A segmentation rule does not run continuously, up to five times, or as many times as the user specifies when they create the rule1. References: 1: Segmentation Rules

The list email or email template report deliverability metric that could indicate an unverified sending domain is hard bounces. Hard bounces are permanent delivery failures, such as invalid email addresses or blocked domains. Hard bounces can negatively affect the sender reputation and the email deliverability. One of the possible causes of hard bounces is an unverified sending domain, which means that the email domain used to send emails from Marketing Cloud Account Engagement is not authenticated and verified by Marketing Cloud Account Engagement. An unverified sending domain can result in the email servers rejecting the emails as spam or phishing attempts. To avoid hard bounces and improve email deliverability, the marketer should verify the sending domain in Marketing Cloud Account Engagement by following the steps in the reference link. Forward count, skimmed percentage, and total queued are not deliverability metrics that could indicate an unverified sending domain. Forward count measures how many recipients forwarded the email to another person. Skimmed percentage measures how many recipients opened the email for less than two seconds. Total queued measures how many emails were waiting to be sent from Marketing Cloud Account Engagement. These metrics are not related to the email domain verification or the email delivery failures. Reference Verify Your Sending Domain