I would personally go with Option 1. The purpose of 301 redirects is to say that a piece of content has permanently moved from one location to another. Therefore the content on the old location should point to the content on the new location, not the homepage.
Google will often not pass link equity if the new page is completely different from the original, which is why redirecting a bunch of domains to a new site isn't going to pass all their link equity to the new site. Like you pointed out, you will also dilute the value of the keyword relevancy.
It's also bad from a user perspective-- if you have a lot of links going to your co-brand, and people are clicking on those links, having them taken to your homepage is a poor user experience, and can result in increased bounce rate. If those visitors are getting to your co-brand through search, it could even be a negative signal for the search engines.
301s should be directed at content that's as similar as possible to the original content, that's the general rule.