[SalesForce] Input placeholder in Visualforce page

You can bind apex:inputText to a variable on your controller. You can bind apex:inputField to an sObject field in your controller When you hit the commandButton the viewstate will be transmitted and the variable assignment will happen, you don't need to explicitly pass in the value as a parameter. Just access the variable in your controller as normal.

For the first approach you can try to use the following code as pattern your implementation. First add a timestamp as a string to the Comment__c field. Then save the record and redirect user to this record:

public PageReference save(){
    // First concatenating the old value with a new one
    custObj.Comments__c = dummyObject.Comments__c;
    // Now adding a timestamp to the new comment 
    custObj.Comments__c += ' ' + Datetime.now().format('MM/dd/YYYY-HH:mm:ss');
    // Upserting the record
    upsert custObj;
    // Redirecting user to the record
    PageReference pRef = PageReference('/' + custObj.id);
    return pRef;
}

And for the next appoach i would create a dummy object where you can temporary hold the original data for the future use:

public Custom_Object__c dummyObject;

// Constructor of the controller
public YourController(){
    dummyObject = custObj.Comments__c;
    custObj.Comments__c = '';
}