RAW Fireball appears to ignore full cover, if there is a path fully within range of the effect that can reach the target, as this spell specifically travels around corners.
If there are no corners this spell can successively travel around within it's effect radius to reach the target, then the target cannot be reached by the effect, and has total cover, causing it to ignore the effect entirely.
Note, there is no limit specified by the spell, stating how many corners can be traveled around, or how arbitrarily complex the path it must follow can be. This leads me to conclude, if you can draw a line that remains entirely within the effect radius that reaches the target, they are hit by the spell.
Also, another thing to note, this is not a point originated explosion as in normal physics, but rather a magical effect that fills any area it can reach with magical flames.
It must be kept in mind that an Antimagic Field only suppresses a spell, it does not dispel it.
What I mean by that is that a spell is not destroyed when it passes through an Antimagic Field. Think of it as magical destructive interference localized on one spot. While the magic cast outside of the sphere interferes with the antimagic in the field, the effect of the spell can't be manifested. However, as soon as the magic passes that field the effects can reappear. This is why summons only vanish temporarily when an Antimagic Field crosses their paths and reappear instantly after.
A creature or object summoned or created by magic temporarily winks out of existence in the sphere. Such a creature instantly reappears once the space the creature occupied is no longer within the sphere.
Now let's check out the list in order:
Spellcaster inside the field. First and foremost, the description of Anitmagic Field explicitly states that spells cannot be cast from within the sphere.
Within the sphere, spells can't be cast, summoned creatures disappear, and even magic items become mundane.
The line of effect passes through the sphere. In this case, the spell is not suppressed in any way, as neither the caster nor the target are in the field and the field does not act as cover from spells.
Spells and other magical effects, such as Magic Missile and Charm Person, that target a creature or an object in the sphere have no effect on that target.
The spells passes through the field. This can be seen in two different ways: an AoE line or a magic projectile like Firebolt. Either way, the answer is the same, which is that it is suppressed while in the field and is no longer suppressed outside of it. For an AoE effect, the area of the spell that overlaps the field is suppressed, which means that a Lightning Bolt would stretch to the beginning of the field, then be suppressed, then reappear after the field as stated in the spell description. A Firebolt would travel in a straight line, also disappear as it passes through the field and reappear as it emerges from the other side.
The area of another spell or magical effect, such as Fireball, can't extend into the sphere. If the sphere overlaps an area of magic, the part of the area that is covered by the sphere is suppressed. For example, the flames created by a Wall of Fire are suppressed within the sphere, creating a gap in the wall if the overlap is large enough.
A spell is cast on a point within the field. The answer is the same as the answer for (3). Although the "target" for the spell is a location within the sphere, the spell itself is not a targeted spell but an AoE spell. This behavior is the same as that of a Globe of Invulnerability.
They would not stack
It's covered by this sentence:
Note, that this is referring to Bonuses, but I believe the same applies to Damage as well, unless the spell says it stacks, it wouldn't stack.
Similar Reference in the Magic Section:
Which I think covers the damage stacking aspect.