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The biggest concern is duplicate content.

If the domain acmewidget.com points to widgetzone.com/acme-widgets and that directory is accessible from the widgetzone.com website then the search engines will see acmewidget.com as a duplication of widgetzone.com/acme-widgets and almost certainly exclude it from the search engines.

I'd recommend setting up microsites under your keyword rich domains that sell a small range of products - the work load is manageable and as long as you provide some unique descriptions and content will work alongside your main website.

If each URL with a different query string displays different information then they are effectively different pages, regardless of whether they're based on the same page framework or not (in this case /10-pounds-products/) - you only need to use a blanket canonical tag like the one' Joost's plugin gives if you want all these pages to be considered the same thing.

If you would like these pages to appear in the SERPs then you should absolutely NOT have a canonical tag telling Google all query string versions of /10-pounds-products/ are the same - not least because it's not true.

In answer to your questions -

1. If you can change it to make whatever query string version you're viewing the canonical URL then do so otherwise I would remove it completely.

2.if you keep the canonical tag then only /10-pounds-products/ will be listed in the SERPs