Magento – HOW TO Change # of products per row, specific for each page layout

In addition to what @benmarks answered:

Here is a little code to check some of the conditions you mentioned:

// Check if it's a CMS page:
$page = Mage::getSingleton('cms/page');
if ($page->getId()) {
    // The current page is a CMS page

    if ($page->getIdentifier() == Mage::getStoreConfig('web/default/cms_home_page')) {
        // The current page is the CMS home page
    }
}

$product = Mage::registry('current_product');
$category = Mage::registry('current_category');
if ($product && $product->getId()) {
    // The current page is a product page.
    // If you only want the main product detail page, also check for 
    // Mage::app()->getFrontController()->getAction()->getFullActionName() == 'catalog_product_view'
    // Be aware that a current_product and a current_category can be set at the same time.
    // In that case the visitor is viewing a product in a category.
} elseif ($category && $category->getId()) {
    // The current page is a category page
    // If you only want the category list page, also check for 
    // Mage::app()->getFrontController()->getAction()->getFullActionName() == 'catalog_category_view'
}

// Check for cart page
if (Mage::app()->getFrontController()->getAction()->getFullActionName() == 'checkout_cart_index') {
    // The current page is the cart
}

To check if the cart is empty or not use

$isEmpty = Mage::getSingleton('checkout/session')->getQuote()->getItemsCount() > 0

Also you asked if you can check the current page inside a CMS block.
That probably is not a good approach (you would have to include custom blocks, which would defeat the purpose of using CMS blocks in the first page).
Instead it's better to check the current page first, and then choose the CMS block to display based on that.

EDIT: fixed Mage::app()->getRequest()->getFullActionName() to be Mage::app()->getFrontController()->getAction()->getFullActionName() as mentioned by @Joseph in the comments. Thanks!

Here is given programmatically adding media images to product, you can adjust this code to your own importing script:

    $mediaGalleryAttribute = Mage::getModel('catalog/resource_eav_attribute')
        ->loadByCode(Mage_Catalog_Model_Product::ENTITY, 'media_gallery');
    $mediaGallery          = $mediaGalleryAttribute->getBackend();
    $attrCode              = $mediaGalleryAttribute->getAttributeCode();


    $product          = Mage::getModel('catalog/product')->load($productId);
    $mediaGalleryData = $product->getData($attrCode);
    /**
     * If you want to remove old images use this code
     */
    if (isset($mediaGalleryData['images'])) {
        foreach ($mediaGalleryData['images'] as &$image) {
            $image['removed'] = 1;
        }
        $product->setData($attrCode, $mediaGalleryData);
    }
    /**
     * Here is real image file path
     */
    $thumbnailFile = 'real/path/to/thumbnail/file.jpg';
    $imageFile     = 'real/path/to/image/file.jpg';
    try {
        //Set thumbnail
        $imageFileUri = $mediaGallery->addImage($product, $thumbnailFile, null, false, false);
        $mediaGallery->setMediaAttribute($product, 'thumbnail', $imageFileUri);

        //Set image (or base image, small image whatever you want
        $imageFileUri = $mediaGallery->addImage($product, $imageFile, null, false, false);
        $mediaGallery->setMediaAttribute($product, 'image', $imageFileUri);
        $product->save();
    } catch (Exception $e) {
        Mage::logException($e);
    }