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The optimal redirect for both visitors and search engines is to keep the structure as it was, meaning #1. The optimal solution also follows your options numbering, meaning that for both search engine and visitors the last options is the least desirable. The optimal solution workload wise, is the exact opposite, as it often is. Depending on how well related the content is, it might be possible to opt for #2 but it is very rare that option #3 would work well as the content of the entire website, including all its subdomains, has to be extremely well related and basically only cover one single topic. A few simple questions might help: Are all the topics of every single page of the forgotten co-brands present on the homepage? If not, then #3 is not a very good option. Are all the topics of every single page of the forgotten co-brands present on the associated subdomain? If not, then #2 is not a very good option. Another thing to consider is the amount of pages that will be re-directed. I actually have a problem with that at the moment, as I am really not sure how well that will be treated by the search engines. IMHO you should look at how the redirects are for people first, bots second and equity/domain authority/etc. last. Not to mention that I think that no matter which group you put first, the optimal solution out of the 3 options stays the same as your numbering anyway.

I presume that the engines simply disregard one of the tags vs. building a penalty based rule, but because we'll also use this solution for canonical tags, there's just too much risk with the approach. I still think it may be a solid (simple) test, so if I manage to get some free time, I'll set something up and post results. Consensus says...don't do it! Thanks for confirming my fears