I Don't Even Know

Buck starts getting used to having a hyperactive toddler at home, develops a routine and rituals with Theo, and really lets himself think about this being the rest of his life. and realizing that raising Theo is the best gift life has ever given him. Buck just knows this love for the boy is here to stay, and he really wants their situation to be permanent.

Except he is a guardian, not a parent, in the eyes of law. And naturally, someone next of kin in Connor and/or Kameron's families volunteers to adopt Theo. Buck is informed of that, and it would be very okay for a relative to adopt Theo — except it's not two days after the car crash, but six months. And he was the one to make a home for Theo and be with him these past months, he was the one who helped him through one of the worst tragedies in a kid's life.

Buck decides to fight for Theo in court, especially when he notices how hesitant the boy looked when he told him about the relative about to look after him.

st. christopher, patron saint of travellers — st. theodore, patron saint of soldiers

"How could you know that?"

"Because he already is"

And Buck like isn't a part of the conversation but overhears it. Then cut to a scene of Chris and Theo meeting and the framing just reenforcing that Chris is Buck's first kid.

Oooh that's a really good point.

I do think it might have been floated around the beginning of the season (what with Oliver mentioning it in an interview), but maybe not seriously considered. And then suddenly, for some reason (Tim watched the Pitt and saw Whitaker helping Amy take care of her son Theo after his father died) around episode 15 he made the decision (per Oliver's interview answer) that the baby would return and they were going to drop any former LI (thus Lou unfollowing Oliver probably because he got the call that they won't need him back).

I think that Tim, after the backlash of 8x06, considered that Bucktommy might have enough shipper support and decided to try something. At the same time he knew he would have to make budget cuts for the next season, so the idea was this:

Send Eddie to El Paso, fix his relationship with Christopher, and have him become a cop there (letting Ryan in the dark for awhile if he would be back ir not). That's why the scene of him and Chris driving into the sunset and the deleted scenes of Eddie becoming a cop or having that idea of it exists.

At the same time, he planned for Ravi to die in the lab episode and Tommy to return (bringing back both characters a little before it).

Then we have 8x11. The episode leaks, and he senses the reaction from the buddie shippers and gives up (Aisha revealing that the episode was to close the door on Buddie, but Tim avoiding to confirm it). Also around the same time, rumors circulate about what Eddie would be doing in El Paso.

Tim drops his plan. The writers then realized they would need more time to rewrite and film the lab episode, so they wrote and reshoot/filmed 8x13 (Invisible) at the same time as 8x14, taking some scenes already filmed with Eddie and the resolution with Chris. 8x14 ended up being divided into two parts, so they could work on Bobby's death as a main character and his goodbye (and save more on the budget).

So I think that for a moment Tim considered leaving Bobby alive and bringing Tommy back so that in season 8, he would end up reconciling with Buck after the lab episode, in which Ravi would die, as well as writing Eddie and Chris off of the show and staying in El Paso. BUT he changed his mind and all possibilities fell apart as soon as he decided that Bobby would die. As everything was done in a rush while they were still filming, some scenes made to set up a bucktommy reconciliation were kept (and because they were good scenes). But after 8x16, Tim only tried to correct the course by sending the characters back to their old positions, with minor alterations due to Bobby's death.

it's a plausible explanation!

Eddie suggests they get married to make the adoption easier/look good on paper?

solely because ryan and oliver would still be weird about each other and that would still bleed into the characters