re: HELP! Can anyone give me advice on replacing bearings on my planer cutterhead?

Get a set of verniers and measure the outer bearing cases, from the IPD the assembly you have provided it looks like it is removed from the driven side, meaning its this bearing that the outer case is bigger than the cutter head body, even if its only a clearance size.
(However from your notes and photos this looks impossible to do)
You will/should consider removing the blades first.
Otherwise lever the bearing case out by applying force to the outer case You could do this this with a drift or a expanding bolt
Either way one of the bearings will move along with the cutter head until it touches the side of the housing.

You could also heat the housing and spray the bearing with freeze to loosen it.
It must come out otherwise how did it get assembled in there in the first place.
Now if this does not work,
On the failed bearing remove the ball bearing retainer case, (this is the metal part equally spacing the balls around the bearing) be aware this is fairly brutal work!

Once you have the race cage retainer out push all the balls together upwards leaving a gap at the bottom, then push the shaft down and the balls will snap away from the inner race, allowing you to remove them.
Remove the balls and then in gap between both inner and outer race insert the gear puller legs and remove the remaining bearing shells.
Now remove the second bearing and there may be enough space to swing the cutter assembly out.

Do not throw the bearing cages away as they have size numbers on them possibly in a code, otherwise take them to a bearing service supplier and they will be able to give you direct replacement, even if its a non stock item and has to be ordered in.