Penetrating the circle has no special effect.
It doesn't matter which side of the circle's barrier the creature is on. As long as either the creature or its target is inside the spell's area, the disadvantage on attacks and the immunity to various mental effects applies. Mechanically, the barrier is only relevant to the first of the 3 listed effects, which makes it difficult to traverse that barrier in one direction.
An inverted Magic Circle has the same area of effect as a "regular" one.
The confusion about how an inverted Magic Circle works stems from a mistaken assumption: that an inverted circle's area of effect is the entire world outside the circle. In fact, regardless of which kind of circle you create, the area of effect is always a cylinder 20 feet wide and 20 feet tall, and as such it has no effect on any interaction between creatures that are entirely outside that area (just like any other area-of-effect spell).
So given this, what does an inverted circle actually do? Let's start with what the spell says about inverting:
When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.
With that in mind, let's rewrite the effects of the circle to be consistent with these changes (rewrites highlighted in bold):
- The creature can't willingly
enter leave the cylinder by nonmagical means. If the creature tries to use teleportation or interplanar travel to do so, it must first succeed on a Charisma saving throw.
- The creature has disadvantage on attack rolls against targets
within outside the cylinder.
within outside the cylinder can't be charmed, frightened, or possessed by the creature.
The only thing to keep in mind here is that as noted above, these effects only apply if an affected creature is either inside the spell's area or attempts to interact in some way with someone or something inside the spell's area. Because the spell's effects are inverted but its area is not, this leads to a few unintuitive asymmetries between a "regular" circle and an inverted one. For example, if you are protected inside a regular circle, a creature that successfully enters the circle (either via unwilling movement or teleportation) still has disadvantage on attacks against you and is unable to charm, frighten, or possess you. However, if you are protected outside an inverted circle, a creature that successfully exits the circle is no longer affected by the spell and can attack you, charm you, etc., completely unimpeded.
In practical terms, this means that an inverted circle provides less protection, since its protection effectively ends once an affected creature is able to penetrate the barrier, not because of any special function of the spell itself, but simply because the creature has left the spell's area of effect. Hence, the only reason you should ever use an inverted circle is to trap or imprison a creature inside it. If your goal is to protect yourself against incoming attacks, taking cover inside a standard Magic Circle will be much more effective.