Google considers this to be spam. Sometimes pages get away with doing this, but generally you're going to eventually get a manual action reported in Search Console.

I generally recommend putting basic Product markup (name, price, maybe image, URL pointing to the single product page) at that level.  The idea here is to let Google understand that that page contains a big list of products that fit the category as seen in the page title.

DO NOT put reviews at this level--I saw something from Google recently that says they consider that to be a spammy attempt to get ratings snippets in the results for that page.  Put the reviews only at the single product page level.

There isn't really any limit, like there is for page titles, meta descriptions, etc.  Typically you'll want the ALT text to explain what's in the image--the original purpose was to show the user what the image was before it was downloaded, and also for vision impaired folks, the screen readers would read to them what was in the image by reading the ALT text.

If you're looking for the image to reinforce the relevance of the page for the page's target topic, then make sure that the topic term is in the ALT text, usually as part of a long phrase or sentence.  If you're looking for the image to rank well in Google image search, then I'd keep the ALT text to just what the target term is (and of course make sure the page title reflects that term as well).

No, keep doing it the way you're doing it.  That's perfectly good link juice flowing between those pages.

Breadcrumbs are a nice way to communicate the hierarchy to Google--not because they're breadcrumbs, but simply because of their nature: all pages at each level contribute link juice back up to each of its ancestor pages.  A child page has the least internal links; its parent has more; its grandparent even more; etc.

I found no examples, sorry...

I don't understand your comment about rel=canonical.  There should be ONLY ONE rel=canonical, and it should reference its own page, EXCEPT in the rare case I outlined above where the content on two different country pages is essentially identical.

Separate the language markup issue from the pagination issue, and treat each of the paginated pages just like any other page on the site.

You should have an hreflang statement for EVERY language page you support for each page in the pagination sequence, including the current page.  So, for example, if we're looking at Italian page 17 of your Purple Widgets category, it should have an hreflang for the Italian page 17, as well as for the English page 17, French page 17, etc.

Rel=next and rel=previous should refer to the page from the same language as the page you're in, i.e. on Italian page 17, rel=prev should point to Italian page 16, and rel=next should point to Italian page 18.

I'm presuming, of course, that the content in the paginated pages is roughly equivalent, i.e. if it's a set of pages of purple widgets that you sort them the same way on the Italian version as the French, etc.  But really, if you didn't....I'd still probably do it the same way.

Don't forget to set the rel=canonicals as well.  Unless you're looking at two pages with the same language and content but targeting different countries (e.g. Portugal and Brazil, with no pricing info on the pages...in that case, you might rel=canonical both the Portuguese and Brazilian pages to one of those), each page will rel=canonical to itself.