People weld cast metal parts all the time. Same for aluminum. The process is just less familiar... aluminum wire is softer than steel so you can't just load it into the MIG and go - you usually want a wire feed gun that puts the spool directly on the back of the MIG gun. And you would usually use Argon shield gas (rather than C25 or C100 for stuff like mild steel).
Tesla's gigacasting cars are roughly as repairable as any other modern car. Lower speed collisions can sometimes be repaired. The repair may not leave it as structurally sound or with the same collision resistance. The older the car gets the more likely it is to be totaled even in such a lower speed collision because these kinds of repairs are labor intensive.
Medium and high speed collisions are always a full loss event. That's a deliberate design decision to sacrifice the car (eg crumple zones) to protect the passengers.
Insurance totaling out Teslas is to some degree an early adopter problem. Parts are less available, service centers with experience may not exist in your area, third party service manuals haven't been published, etc. These things make the cars more expensive to repair. All of these issues are rapidly improving but it should be obvious that they aren't a 100 year old car maker so their network of spare parts and repair shops will smart out smaller.