The spacer gives the all up total clamping dimension for the non rotating components within the wheel design, so stays still with the frame spindle etc. If the spacer is wrong in dimension it'll either pinch the bearings and preload them if too short, or allow the wheels to ultimately move sideways if too long.
So dimension is critical, how it's held is as you speculate as convenient to prevent it dropping out of alignment when the spindle is removed. I guess the original is just the cheapest and simplest they found to do it, but the placement is ultimately controlled by the spindle going through there. Making location just convenience.
It doesn't really matter if there's no "inside" face seal on the bearings as nothing really gets in there anyway.
You'd probably swear at it if you couldn't get the spindle through, well I would anyway

depends then how confident you are in the loctite solution. It is more shear prevention (two surfaces sliding past each other) rather than "glueing" in position two components, so not ideal in doing what you want.