[SalesForce] Data Migration from one production org to another

What do you mean by the FeedComment object is not accessible. You should be able to access it.

In an execute anonymous window try this

 list<FeedComment> fComments = [Select Id, FeedItemId, CommentType, CommentBody From FeedComment limit 25];
 system.debug(fComments);

What results do you get.

You should be able to extract the feedComment objects and reparent them in the same way that you have described above. Where exactly are you hitting an error?

In my opinion, it is better to achieve the migration using the Data Loader, when you can, than it is to write Apex, but as with everything it depends on the specific scenario.

Benefits of using the Data Loader:

• You don't have to worry about coding errors when you use the Data Loader.
• You don't have to worry about limits as much. For example, if you are migrating a sizeable number of records you'll quickly run into limits with the number of records returned by your SOQL query.

That being said, there are some times when Apex might be the way to go.

• You have complex business logic that isn't as simple as exporting one Object's fields and importing to another. In this case, I'd recommend creating a Batch Apex job, unit testing it, and testing it in a sandbox prior to production.
• You have an Object that has a few (e.g., 10) records and you are copying a value from one field to another field. Anonymous Apex would be reasonable to me. With this small data set, it's simple, easily verifiable, and you likely won't hit limits.