SEO and Permalinks Redirection Question

I have a client that has manufacturers products that are often bought by a new manufacturer and the manufacturer name and/or the product name changes. We have not launched the new WP/WooCommerce site yet and are still thinking through the best ways to redirect all the old site pages to the new, while not having to update them in the future.

I have tested changing the current page name/slug from:

http://www.domain.com/shop/category/subcategory/old-manufacturer-productname/

to:

http://www.domain.com/shop/category/subcategory/new-manufacturer-productname/

I can use either of these links and get to the new page name:

http://www.domain.com/index.php?p=####
http://www.domain.com/shop/category/subcategory/old-manufacturer-productname/

1) Will WordPress always track the name change?

2) If we are redirecting from the old site links, for example,
http://www.domain.com/prod.php?product_id=816, will anyone following an old link end up at the new page, even if the name changes… could be several times?

3) Is there any problem, especially with SEO, with redirecting the old site pages to the ugly permalink? And will that always end up at the current page name?

4) Could this cause too many redirects and hurt in the search engines?

5) Is there a “best way” to do this with little to no changes to the redirects in the future?

THANKS!
Julie