Can I transform a value during import (mark up prices, combine columns, set stock status)?

Yes. On any field you choose either direct CSV mapping OR a PHP function, not both, because the function is the mapping: whatever it returns is stored for that row. The function receives the entire row, so you can read any column with $data[$h['Column Name']].

return function($data, $h) {
    // add 30% markup to the retail price
    $price = $data[$h['Retail Price']];
    if (!is_numeric($price)) {
        throw new InvalidArgumentException('Retail Price must be numeric.');
    }
    return (float) $price * 1.3;
};

Validate numeric input before doing arithmetic. A cast can discard a decimal fraction or trailing text; use a conversion that matches your data. To combine several image columns into one gallery field:

return function($data, $h) {
    $images = [];
    for ($i = 1; $i <= 6; $i++) {
        $key = 'image_' . $i;
        if (isset($h[$key]) && !empty($data[$h[$key]])) {
            $images[] = trim($data[$h[$key]]);
        }
    }
    return implode('|', $images);
};

Further reading

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Written for Super Speedy Imports 2.96.0.