Crack/fissure in the headtube
Q: "[W]ould a competent welder be able to fix that crack[?]"

No, that frame is done for. By attempting to weld such a crack, a welder would prove their incompetence.

Even if you fix that crack you might find another crack on the right hand side, mirroring this crack. What is more, the crack will go considerably deeper.

I had a crack at the same spot and we asked the manufacturer (Corratec). According to their frame builder welding would warp the head tube too much to seat the head set properly.

Q: "[Why] can a crack develop at that point[?]"

Aluminium is susceptible to fatigue under light dynamic loads.

The upper bearing of the headset is pressed into the headtube at this point.

The position of the crack suggests it is caused by a force pushing the upper bearing forward. Due to the great mechanical advantage of the fork/headset system forces pushing the wheel backward require the upper support to bear a force pointing forward. This strains the upper support. (The lower support is compressed).

When you pull a ring from the inside it will typically break at the same points your tube cracked. (If I remember correctly from undergrad engineering ±2/3π from the direction of the force.) (Try it for example with a bagel.)

If your headtube showed cracks at any other place it would suggest machining tolerances not met. A head tube that is too tight cracks when the upper bearing race is pressed in. These cracks would then grow, for example, due to stress corrosion cracking.

Q: New frame?

Have you tried to ask Ghost? Corratec have given me a new frame when their ten year old frame failed in the same mode.

PS: Do not ride your cracked bike! Not even a single metre: it might fail catastrophically without warning.