Possible to use bind :variable inside SELECT clause?

You have to use dynamic SOQL if you want to add fields dynamically. Merge syntax is not supported.

List<String> fields = new List<String> { 'Id', 'Name' };
String soql = 'SELECT ' + String.join(fields, ',') + ' FROM MyObject__c WHERE ...';
List<MyObject__c> records = Database.query(soql);

I actually often have dynamic filters as well when I'm in such a scenario, and might use a similar strategy there. Another basic example:

List<String> fields = new List<String> { 'Id', 'Name' };
List<String> filters = new List<String> { 'CreatedDate = TODAY', '...' };

String soql = String.format('SELECT {0} FROM MyObject__c WHERE {1}', new List<String> {
    String.join(fields, ', '), String.join(filters, ' AND ')
});
List<MyObject__c> records = Database.query(soql);

You'll need to use a dynamic query for that: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_dynamic_soql.htm

Here's an example from the article to show a bit about how it works:

String myTestString = 'TestName';
List<sObject> sobjList = Database.query('SELECT Id FROM MyCustomObject__c WHERE Name = :myTestString');

Basically, you'll just update it to look more like this:

String myTestString = 'TestName';
List<sObject> sobjList = Database.query('SELECT Id, ' + ss + ' FROM MyCustomObject__c WHERE Name = :myTestString');

Salesforce does not support variable binding in the SELECT clause.

From Using Apex Variables in SOQL and SOSL Queries:

SOQL and SOSL statements in Apex can reference Apex code variables and expressions if they’re preceded by a colon (:). This use of a local code variable within a SOQL or SOSL statement is called a bind. The Apex parser first evaluates the local variable in code context before executing the SOQL or SOSL statement. Bind expressions can be used as:

• The search string in FIND clauses.
• The filter literals in WHERE clauses.
• The value of the IN or NOT IN operator in WHERE clauses, allowing filtering on a dynamic set of values. Note that this is of particular use with a list of IDs or Strings, though it works with lists of any type.
• The division names in WITH DIVISION clauses.
• The numeric value in LIMIT clauses.
• The numeric value in OFFSET clauses.

Bind expressions can't be used with other clauses, such as INCLUDES.