I am unsure on some points and am happy to be corrected, but..
1: Both. The ligament may regrow and mesh at the sutured point, may regrow and secure a surgilig tether, or may be too damaged for use in reattachment. An artificial tether is often used in conjunction with the sutures in order to relieve the load on the healing tissue (or used to completely replace the tissue in cases where required). This is often a dynamic decision made by the surgeon, as sometimes a scan may show tissue to work with, but on inspection it may be deemed unsuitable.
2: The excision of bone is cited as ‘to prevent the distal head of the clavicle from abrading against the acromion’. This is because the clavicle will exhibit increased fore and aft movement as there is not reliable method of fixing the distal end in position due it being a naturally flexible connection.
3: The few weeks post injury are often enough to help the (now slightly longer due to the stretch and tear) ligaments to mesh and heal slightly. With the clavicle now offering much less of a supportive role, these ligaments are generally under less load than they used to be.
4: That was my choice. With increased muscle mass comes much greater stability.
5: My initial surgery was a few months after the accident and I had made little progress. This was due to the complete detachment of the clavicle and having to deal with a good amount of soft tissue damage and other injuries at the same time, limiting rehabilitation. In cases where there is still a good, albeit stretched and damaged attachment, (as I have ended up with after surgical failure, effectively leaving me with a grade 3/4) then rehabilitation was fairly quick.
Even if you go back in a few months and the ligaments are past it, you are just back in the same position as many start in. There are a good few options for these cases, both artificial and natural harvested.
A great source of info run by my surgeon: https://www.shoulderdoc.co.uk/