Looking at the details of the family profiles, there was a recent merge. In fact, Patrick Horan has 2 different 1911 censuses attached.

Either Patrick married twice, first to Ellen McGrath and second to Johannah Cooke, or two men and their families have been conflated.

I looked first at the 1890 civil marriage for Patrick Horan and Johannah Cooke. So far, so good as Patrick is listed as "widower."

Ellen McGrath Horan's profile has a death of 1889, fitting with that 1890 2nd marriage for Patrick. But, there is a 1911 residence for Ellen, confusing the algorithms.

There are 2 unrelated people living in the household in 1901, and that causes the algorithm to suggest we check to be sure everyone is connected. The 2 people, Henry Regan and Margaret Ryle, are servants, so we can dismiss the suggestion that something has been overlooked. And, again in 1911, there is an unrelated servant, John Connor, so that suggestion can also be dismissed.

Hope this helps.
FYI - the community has been having issues with image attachments. The easiest way, for now, is to clip the image with the Windows Snipping Tool or your operating system's equivalent.

On that 1901 census, the Horan family on the 1901 census are all already attached to the correct (I hope!) profiles - you can tell that because they are in green and there is a Detach link.

The guys from that 1901 form who have not been attached are the ones in white at the left - Henry Regan and Margaret Ryle. Expanding the full version of the 1901 census where it's attached to Johanna, shows that Henry and Margaret are servants, which is why they've not been attached to the Horan family - because they're not family members.

To answer your specific questions:

(1) There is no button to attach them because they are either already attached or cannot be attached to that family.

(2) The Detach links will detach the attached source from the person. You don't want to do that, they aren't the problem.

(3) Why is it asking you to attach the source? It's not asking you to attach the source to the Horan family, it's asking you to attach the source to the profiles for the 2 servants. If you don't know anything about them, do not do anything about that - it's probably too difficult, I suggest, to find their profiles, if they've even been created. I certainly wouldn't touch them - I don't know anything about them, and I might easily find the wrong Henry Regan, etc.

So why is this facility there? Basically because there are times when the unattached person in white on the left is on the right as a family member but their census has not been attached. This can easily happen if it's a daughter who's changed her name on marriage so no match has yet been made because it's not obvious. Should you attach the census in those circumstances? Only if you have done the hard work to satisfy yourself that the unattached (say) Susan McLaren on the left really is the same as the Susan Horan on the right who hasn't already got a census attached to her. (Names made up…)