Again, that's a lot of assumptions. It sounds like you are describing a haunted castle with hidden rooms.
You are making at least as many assumptions as anyone else here.
1) You're assuming that the same stage that you observed in 2018 was present in 2006.
2) You're assuming that because you were able to crawl under the stage unobserved with three or four other people in the bar, Brian would have been able to do so in a crowd.
3) You're assuming that even if a person was stealthy enough to crawl under a stage unobserved in a crowded bar, that same person would have been incapable of enough stealth to squeeze through a loosely chained door unobserved.
4) You're assuming that the cops who had their backs to the construction exit could not possibly have missed a person capable of enough stealth to crawl under a stage in a crowded bar squeezing through the loosely chained door of that exit.
5) You're assuming that there could not possibly have been any other access point to the construction site.
6) You're assuming that Brian did not visit the construction site because tracking dogs did not detect his scent, yet somehow these same dogs can be excused for not detecting his scent under the stage where he allegedly hid for at least thirty minutes, even though he would have been shedding the microscopic particles that tracking dogs detect all the while.
7) You're assuming that LE had assistance from construction workers while searching the construction site.
If I took the time to go back through your posts , I could probably find more.
You're also choosing to ignore important evidence:
A. The hideous stench mentioned by patrons at the theater complex next to the bar.
B. The strong parallels to other cases in which missing people were found to have died from positional asphyxia after becoming lodged in obscure crevices, chimneys, or air ducts.