One important question that I'm unclear on is how long it takes to fix one of these cables. If it takes months then that is quite a wide window in which an attacker could incrementally take down cables.
Generally it can be fixed in days. They raise it from the sea floor and splice in a new cable section.
True but ships and crews with the equipment to do the repairs are limited. It's possible to overwhelm the repair capacities. Also, it takes time to physically travel between cuts so while cuts in the Baltic might take a week or two to fix, a cut in the Atlantic and one in the Baltic may take a week or two just for travel.
If someone will try to overwhelm the repair capacities for integral communications, they'll be dealt with like pirates - simply sunk and be done with.