To determine the best identifier for subscribers across all channels, Contact Key (A) should be recommended.
The Contact Key is a unique identifier that can be used consistently across all channels, ensuring a unified view of the subscriber's interactions and data.
References:
Salesforce Marketing Cloud Contact Builder
Contact Key Overview

To validate an email address prior to form submission, the REST API endpoint /address/v1/validateEmail can be used. This endpoint allows you to check if the email address is formatted correctly and potentially validate it against known email address patterns.
Salesforce REST API for Email Validation

To remove line breaks within the Comments field and replace them with a single space, you can use the REPLACE function in combination with the CHAR function. The CHAR(13) and CHAR(10) functions represent carriage return and line feed characters, respectively.
* REPLACE Function: This function is used to replace occurrences of a specified string with another string.
* CHAR Function: This function returns the character based on the ASCII code provided.
* Example:
SELECT REPLACE(REPLACE(Comments, CHAR(13), ' '), CHAR(10), ' ') AS Comments FROM YourDataExtension Salesforce SQL Reference

To get a list of subscribers who were sent an email within the past 12 months, you should query the Job and Sent data views. These data views contain detailed information about email sends and can be used to filter subscribers based on the send date.
* SQL Query Example:
SELECT s.SubscriberKey FROM _Sent s JOIN _Job j ON s.JobID = j.JobID WHERE j.SendDate >= DATEADD(month, -12, GETDATE()) Salesforce Data Views

The Balance field is a text field and contains the value "S6.96", which is a string. The IF statement is comparing a string with a numeric value (6.00). Since the operands are not of the same data type, this can yield unintended results. To correctly compare the balance, the string should be converted to a numeric value.
* Example:
%%[ SET @Balance = "6.96" /* Simulating text field value */ IF (Value(@Balance) > 6.00) THEN SET
@Result = 'Balance is more than $6.00' ENDIF ]%%
Salesforce AMPscript Data Types and Conversion