Looking through the rules, it seems that by a lawyerly reading of MCaE, the "protection" version of the spell (as opposed to the "prison" version; more on this in a moment) essentially has two effects: a mass PoE, and a circle that certain summoned creatures cannot enter.
The mass-PoE effect seems to be essentially unaltered from the original spell, except that it only applies to creatures inside the circle. If you walk outside, you lose the effect; if you walk back inside, you get it again. This makes the personal protection against bodily contact a moot point. One could argue that it technically still exists, but it only applies while you are in the circle, where there is already another barrier in place. Because the circle is not linked to you specifically, it does not collapse if you reach across the circle to attack.
Speaking of reaching across the circle, both PoE and MCaE are very explicit that they only apply to the creatures themselves. Ranged attacks are fair game. Even an imprisoned creature, in the "prison" version of the spell, is explicitly stated to be able to make ranged attacks across the circle, as long as they don't target the circle itself. So yes, creatures inside an MCaE can make ranged attacks across it, but so can creatures outside an MCaE.
- No, you cannot target a point that is blocked by total cover.
From Players Handbook, Spellcasting, subsection "Targets":
To target something, you must have a clear path to it, so it can’t be behind total cover.
If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
When you cast an area spell, you can place its point of origin on your side of a wall, but not on the opposite side of the wall.
- No, area effects normally do not go around corners (unless the spell explicitly says it does).
From Players Handbook, Spellcasting, subsection "Areas of Effect":
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover.
Lets say you cast Spike Growth and put its point of origin near a wall, at a location that isn't behind total cover. The spell's radius would extend in all directions from its point of origin, but it would stop at the wall. The ground behind the wall would be unaffected.
This is the cast for most spells. However, there are exceptions (such as Fireball) that specifically mention going around corners. In those cases, the spell's text overrides the general rule for other spells, and so that spell's effects will not necessarily stop at a corner.
Best Answer
For the normal casting of Magic Circle, it follows the target and people only gain the benefits of the spell if they are within 10 feet of the caster. The protection is an "emanation" which means a continuously radiating effect. Other emanations are detect magic, zone of truth, silence, etc. - none of these affect things that have been affected and then left the area.
Yes, the emanation follows the target as it moves. Again, that's the way all these spells work (silence targeted on a creature being the canonical example). Nothing about the spell (or other spells with the same stats) says it doesn't move with the targeted creature.
For the "binding" version, no, the drawn circle does not follow the target - it's a physical circle drawn on the floor, not part of the spell effect. The whole point of the second version is to keep something from moving.
Really the second use of Magic Circle is an example of poor spell design; it's actually a completely separate spell that overrides a lot of the normal spell stat block (like you don't have to touch the summoned creature, the spell text makes it clear that it gets applied 'for free' with the summon/bind)...