Bung on top of crossover pipe Y

[Ed81DMC]

I’m replacing my left exhaust manifold gasket. Anybody know what this “bung with a bolt in it” is for?
See photo.
Also what is the bedt tool to loosen the upper left (driver side) of the 4 nuts attaching the crossover pipe to the exhaust manifold? It squarely blocked by the oxygen sensor! I’m thinking of using a 17mm crow foot because a 17mm socket won’t fit over the nut without removing the oxygen sensor - which I can’t remove.

What is the best tool to remove the oxygen sensor (if I have to remove it)? It points toward the side of the vehicle and a longer sensor socket won’t fit over the sensor. Is it 13/16” or 20mm or 21mm?