Without Pinbacker, what direction do you think the plot should've gone?

The bad guy should have been the sun, not some random Freddy Krueger.

Don't know. Somewhere else.

Schrodinger's cat walks into a bar, and / or doesn't.

First off I think the film is excellent but I do think having Pinbacker in the last act makes it like so many other films wheras I would have prefered just the space stuff. So here would be my ending with plenty of problems I am sure:

The exploration of Icarus I is the same, except Pinabcker is also dead (after having sabotaged the mission), the bridge between the ships is damaged just by bad luck, Searle still has to stay behind and Harvey still doesn't make it. Trey has still commited suicide.

Once back on the ship the oxygen is much lower than in the actual ending with only enough to get one person to the payload delivery. Obiously the one person is Capa as Mace quickly points out. After a brief argument everyone agrees staging a 3 person suicide pact. At the last minute Cassie backs out. As he is dying Mace tells Capa that the only way to complete the mission is for Cassie to die. Private moment when Capa talks to Cassie telling her that they are all dead anyway. She makes him promise to take her to the surface of the sun then allows him to take her life. Capa kills her and carries her body to the payload. Bomb can still only be set of manually (just because). Cut to amazing scene as Capa sets the bomb off (just without Pinbacker bits).

Slightly different ending would be that Capa sets the bomb off remotely, Icarus informs him that life has survived in the oxygen garden (many more plants), which may be enough to get him home. Cut to observationvation room with filter up, single cremated body.