The three options seemingly were:
1) no surgery and long rehab;
2) repair surgery (internal brace) which is a ~6 month recovery; or
3) reconstruction surgery (Tommy John) with a 12+ month recovery.
(1) was never really on the table. Most reports I’ve read indicate that option (2) is the most likely currently. The repair surgery would make him able to start throwing/rehabbing after about 3 months, with an expected 6 month total recovery. (3) would be very bad for Brock for next year, virtually guaranteeing he would not play.
From what I’ve heard, it sounds like the question is whether it’ll be (2) or (3). The 49ers want (2) because it means Brock can start next year. IF Tommy John is really going to be better in the very long run, perhaps Brock would lean that way. This is why he’s waiting for swelling to go down and to get second opinions. Determining whether the repair surgery will be sufficient long term (as compared to the reconstruction).
If it’s a repair, Brock probably starts next year. If it’s a reconstruction, he likely rehabs next year.
I don’t know enough to know if there are significant differences long term between the two surgeries. Because these injuries are rarer in football, there probably isn’t a ton of data out there. Nick Mullins, former 49ers QB, had the repair surgery at one point successfully.