I want to try to defend a reading of this that sees it as a logical must, with the qualifier that it is not necessary to always see Sisyphus happy. I think even a normative reading that insists that we should always see Sisyphus as happy is mistaken.

We see Sisyphus as being condemned to a fate which he is conscious of.

Sisyphus, proletarian of the gods, powerless and rebellious, knows the whole extent of his wretched condition: it is what he thinks of during his descent.

In particular, I think the consciousness aspect is the key part throughout for Camus. The fact that Sisyphus has time to think about his condemnation is important, and the analogy is that the regular human being has the ability (when not busy with menial tasks) to really think about the utter meaninglessness and purposelessness of their existence. It isn't just that life is absurd, but also that we think about this absurdity and maintain consciousness of it.

Tragedy in particular comes from the conscious realisation of the absurd.

The workman of today works every day in his life at the same tasks, and this fate is no less absurd. But it is tragic only at the rare moments when it becomes conscious.

Camus is very interested in this part of the myth. That part where Sisyphus is walking back down the mountain, after the rock has fallen. At this point, Sisyphus is fully conscious of his absurd face and has 'an hour' to think about it. The return in between the automated, repetitive, never-ending labour is what Camus is after. Now, here's where I think it becomes logically possible to see the Sisyphus as happy.

There are repeated moments (infinitely many in the myth) of Sisyphus becoming conscious of the absurd. For every act of pushing the rock up the hill, there is always the descent back down towards earth, where Sisyphus has the opportunity to contemplate his fate. Camus wishes to explore what happens after repeated instances of this phenomenon.

If the descent is thus sometimes performed in sorrow, it can also take place in joy. This word is not too much.

This is the logical possibility part that I want to stress. But Camus goes further by detailing the process that supposedly happens when the absurd is made conscious repeatedly.

When the images of earth cling too tightly to memory, when the call of happiness becomes too insistent, it happens that melancholy rises in man’s heart: this is the rock’s victory, this is the rock itself.

The last few paragraphs cover happiness more in detail, and I am unable to go into detail about them right this second, but I hope to be able to return to give a better response to the last bit. Basically, I think that Camus wishes to connect happiness and the absurd somehow, but not via logical necessity. At the same time, an absurd fate does not forbid happiness, so it is logically possible to be happy and understand the absurd condition.