[BeckyEbook]

Ad 1.
a) The number of items displayed in the table corresponds to the number of files selected before Rename RegEx was selected. Personally, I only select the files whose names I want to change....
b) I think it is possible to reconcile both sides.
Changed elements can be highlighted in the table with a background colour, for example. (Below is an example, but you have to take light/dark mode into account).

Spoiler:

    foreach(QString bpath, bookpaths) {
        QString apath = newbookpaths.at(r);
        ui.tableWidget->setItem(r, 0, new QTableWidgetItem(bpath));
        ui.tableWidget->setItem(r, 1, new QTableWidgetItem(apath));
        if (apath != bpath) {
            ui.tableWidget->item(r, 1)->setBackground(Qt::darkRed);
        }
        r++;
    }

Ad 2.
This line (add space after comma).

Although I have checked this, it is worth investigating further.