To answer your questions:
1. You can use qRules' Insert command to insert rows into a repeating section using rules. Then you can turn off the "Allow users to insert/delete rows" option in the repeating section.
2. Unbound controls are essentially useless. If you don't want the comment input field in your main data source, you can create a simple XML file, like:
<?xml version="1.0" ?>
<formLogic>
<comment></comment>
</formLogic>
and import this as a secondary data connection, then use a textbox bound to the comment field.
3. Using rules to copy a value into a single row of a repeating section requires a little bit of tricky maneuvering. Hilary Stoupa's blog explains how to do it. Where she uses something like:
my:items/my:baseCost[../my:itemType='Labor']
to select a row with a certain value in one of its fields, you can use the last() function like:
prefix:repeatingGroup[last()]/prefix:comment
(using the correct node names of course) to modify the comment field in the last (most recently added) row.