use  the_title();  for the H1 tag instead

Not sure what that other code does... if(!wp_title("",false)) { echo bloginfo( 'title');} ?>

if no title set, show blog title? what pages would this affect?

Yes, that would do good. Since content is identical for each of these products, there should only be 1 URL with all of the variations of that product in order to consolidate all of the authority. If you want to keep all of the variations in search, look into creating anchor links that point to the same "master" url. e.g. http://www.prams.net/easywalker-mini-buggy-lightweight-union-jack-b can be linked as http://www.prams.net/easywalker-mini#union-jack

That way, the URL is the structure is more SEO friendly but aesthetically the site is identical.

edit the HTML in the child theme files. Likely the header and footer files specifically.

You should remove it if possible. No reason to link to a dead link (poor ux, waste crawler time) or call a non-existent resource (slow site down).

However, I don't think it will make any noticeable effect on your rankings.

I would guess similar to 301 redirects... about 2 weeks.

The faster you can get Google to recrawl the pages and index the new canonical URLs, the quicker the authority will transfer over.

Agreed, the redirects/canonicals should be permanent (well, for as long as you want the authority to pass along).

You would see changes in serps within 2 weeks usually.